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VHF AM?

 
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VHF AM?

Post by colly0410 » Tue Jul 29, 2014 5:28 pm

I've been yabbering to my mate Jack again, (the one who said the UK should have used a modified system N in the post I put in the TV section) & he said "we (the UK) should have used wide bandwidth AM on VHF band 2." I thought "hmm, not sure I agree with that" but nodded politely. To push home his point he said "remember how good 405 sound was with a good speaker!" & from what I remember it did sound good. I never thought to ask him how VHF AM would have coped with stereo, I wish I had, would the pilot tone sub-carrier system have worked on AM?

 
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Re: VHF AM?

Post by Kalee20 » Tue Jul 29, 2014 5:39 pm

At a basic level, yes it would. The audio L+R signal, the 19kHz pilot tone, and the L-R audio signal amplitude modulated on the 38kHz sub-carrier (which is suppressed) are just added together giving a composite signal with bandwidth 20Hz - 53kHz. This is used to modulate the RF. Whether this is AM or FM is not relevant. Either will convey this composite signal.

However, whether AM would allow sufficient noise rejection is another matter. Wide-band FM does give better noise immunity, and better rejection of adjacent channels, as the VHF AM versus FM trials demonstrated, so i understand.

 
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Re: VHF AM?

Post by colly0410 » Tue Jul 29, 2014 6:23 pm

Thanks for info Kalee 20. I was trying to think if pilot tone would work on AM, & of course it would be like a colour sub-carrier on a TV signal, thicko muggins here didn't think of that..

Jack is a bit like an electronics version of Fred Dibnah with a north Nottinghamshire accent instead of Lancashire, he loves living in the past with his own opinion on things, great bloke though. Can't wait to meet him again..

 
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Re: VHF AM?

Post by Refugee » Tue Jul 29, 2014 6:36 pm

Back in the 1980s a fellow worker had a valve radio that had VHF AM on the front dial. He put it in the workshop as a works radio.
Would this have used a very narrow band detector coil to produce audio from an FM signal with a single detector diode?
It just looked like a normal 1950s set to me in all other respects. I can't remember the make or model though.

 
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Re: VHF AM?

Post by Michael Watterson » Tue Jul 29, 2014 9:16 pm

110 MHz to 137MHz uses AM.
Two reasons:
1) No capture effect. You can hear two transmissions on same frequency, this is rare on FM.
2) Possibly Doppler. Though this is contentious.

Very Narrow band FM has no big improvement over AM on noise. So Broadcast uses Wideband FM. Mobile radio uses narrow band or even phase modulation. But it's significant that only Aircraft use AM VHF. Some early Mobile low band used AM. For Mobile the NBFM allows better transmitter efficiency and easier transmitter design.

On Broadcast FM you have two effects.
1) Noise isn't linear with signal level. on AM it is. So with FM most people have better SNR. FM at fringe reception "falls off a cliff" and rapidly gets noisier in a linear fashion.

2) Capture. A stronger signal, especially at a slightly offset frequency will "mute" the weaker station. On AM you'd annoying hear loud bits of weaker station during quieter bits of stronger one, or if not at the same frequency you get a beat or whistle due to frequency difference.

The only reason to NOT use FM for all Analogue radio is the Bandwidth. A typical Wide band FM channel is width of entire Long wave (though you could fit 3 channels) and you could only have 3 to 9 channels on MW!

So VHF-FM was the correct choice. Pye wanted VHF-AM, so the BBC did do some VHF-AM tests. But everyone thought they were nuts and VHF-FM Radio was already well established in USA and Germany at least by 1949. Belgium, Netherlands and Denmark also using VHF-FM before UK. Possibly Italy? Philips and German companies even had portable battery valve AM-FM sets before UK launched FM, enabled by DC90. The DC90 was replaced by DF97 in 1955-1956 I think, but used the same circuit. I've tested swapping them.

The only reason 405 was AM sound was to reduce complexity in 1936. If TV had not restarted till 1949 or 1950 it likely would have been FM sound and more lines.
The USA decided on FM for TV sound in 1941. They had used low band VHF-FM radio before 1939, but switched VHF-FM Radio to BandII in 1945.

I think most 1949 - 1950s 625 TV used FM sound too.

Stereo wasn't a consideration till late 1950s, though demos started as early as 1924 (two transmitters and Radios). Current AM Stereo is rather more clever than the Multiplex or Pilot tone 19kHz + 38KHz DSBSC, which is a bad choice for AM. There are two main ways to do AM Stereo:
1) Essentially AM = L+R and phase modulate carrier for L-R. Motorola C-QAM The "winner"
2) use essentially USB and LSB and a pilot at the carrier. Only partially AM receiver compatible. Kahn-Hazeltine

In the USA rather than DAB, they have the misnomer HD Radio, which is just Digital Radio. Most USA "HD Radio" chips can do C-QAM AM-Stereo, though a particular radio model might not implement it.

See http://en.wikipedia.org/wiki/AM_Stereo

My Sony ICF2001D doesn't do AM Stereo, but the IC it uses for SSB (USB or LSB) and synchronous AM to avoid heterodyne whistle by using LSB or USB on AM was a chip for AM stereo.

 
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Re: VHF AM?

Post by Terrykc » Tue Jul 29, 2014 9:59 pm

Michael Watterson wrote:... So VHF-FM was the correct choice. Pye wanted VHF-AM, so the BBC did do some VHF-AM tests. But everyone thought they were nuts and VHF-FM Radio was already well established in USA and Germany at least by 1949 ...

I knew that the BBC did comparative tests in the early 50s but not the rest of the history. Thanks, Michael.

 
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Re: VHF AM?

Post by Michael Watterson » Tue Jul 29, 2014 10:48 pm

"The Setmakers" has a bit about Pye & BBC and BREMA (being a BREMA project) relating to the BBC tests. UK launch of VHF-FM was triggered by South East Third Program interference on MW AM.

Germany lost many AM allocations to Russian and Western Allies Propaganda & Forces stations in 1945. So VHF-FM 1949 roll out was VERY fast and popular as unlike UK 1955-1971 it added many "missing" stations. The UK had much less Radio variety after WWII than in 1930s hence popularity of AFN, Luxembourg etc. In UK the VHF-FM roll out was very slow after South East and was only a simulcast of Home Light and Third. Generally only HiFi buffs for Third bought VHF-FM sets in 1950s and 1960s UK unlike USA and Germany. Oddly though the US doesn't seem to have had portable VHF sets till transistor era. But they did make some very advanced valve VHF sets with nuvistors.

1945 to 1971 was characterised by ossification and maintaining the Status Quo in UK Radio, apart from the panic introduction of R1 in 1967 (average age of listener now over 30). It's nearly a mystery that ITV was created. Also having done that why didn't they create a Commercial Radio outfit two (maybe a national and regional) to have the UK advertisers spend in UK instead of Continental Europe aimed at UK at night and boost Radio sales by having two new channels on VHF-FM. That's what they should have at least done in at least 1960s instead of timeshare Light to R2 & R1 and kludged extra BBC R1. BBC R1 wasn't on VHF-FM till until 1988 -- Crazy! 1971 was (re)start of BBC Local /Regional Radio which had existed till 1939 and was certainly significant 1926 (e.g. Radio Nottingham). UK Commercial radio instead of starting in 1955, only launched about 1973.

Partially the issue was UK Services such as Police, Ambulance, Fire using 97 to 108. The US used up to 108 from 1945 and Germany up to 100 in 1949, later in 1950s up to 104. UK only used 88 to 95 in 1950s & 1960s!

I forgot to mention, but most people know that 110MHz to 137MHz approx is Air Band.

 
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Re: VHF AM?

Post by Synchrodyne » Wed Jul 30, 2014 12:25 am

I too had pondered the original question about the use of stereo subcarriers with VHF-AM. On the face of it, it should have been possible. Given that AM noise has a square shape, not triangular as in FM, then the significant reduction of signal-to-noise ratio that occurs with FM at subcarrier frequency does not occur with AM. On the other hand, VHF-AM has a higher noise floor than wideband FM. Impulse noise reduction might be more difficult, in that noise limiters would need to respond only to noise above 53 kHz, which might make them less effective. I suppose though that with frequency division decoding, a lower frequency noise limiter could be used in the baseband L+R channel.

Anyway, I recently stumbled across a form of proof-of-concept insofar as that in the mid-1950s, the French had dual-language TV sound in Algeria with System E, which used AM sound. I included the scant information – plus some deductions – on a recent posting in another forum, but will repeat here if wanted.

Of course the whole VHF-AM saga smacked more of industrial politics than investigation of valid technical options. The BBC had decided in favour of FM by 1945, and had wanted to start the Third Programme as a VHF-FM only service. As Michael W. has said, Pye was the chief antagonist when it came to FM, and used BREMA as its voice, thus more-or-less forcing the powers-that-be to take notice. In fact it was something of a three-system debate, involving FM, AM wide (allowing the use of impulse noise limiters) and AM narrow options. Apparently Stanley of Pye had a second preference for AM narrow, his first preference having been no VHF sound broadcasting at all. In the end technical sanity prevailed, and the TAC recommended FM and the Government/PMG accepted this. One of the reasons was that with AM, the capital and running costs of the transmitter network would have been very much higher. At equal radiated powers, VHF-AM, with its higher noise floor, had a shorter range than VHF-FM, so more transmitters were needed. But even when the usable signal ran out, the carrier still had the ability to cause co-channel interference at what might be called FM-like distances. Add that to the lack of capture effect with AM, and it limited channel re-use possibilities as compared with FM.

WW 195311 p.531.jpg


The BBC was in a bit of a no-win situation in all of this. It was being beaten up – questions in the House and all that - because many were suffering from poor reception MF AM conditions in some parts of the country, particularly at night (I think East Anglia was one such area), but going to VHF was the only workable option to solve that problem. Thus the VHF-FM network was conceived on the basis of providing interference-free reception across virtually the whole country. The high quality audio aspect was downplayed, in part because the BBC would need to use the existing landline network, which for the most part did not get past 10 kHz, sometimes less. The BBC was then criticized for not aiming higher in audio quality terms. Then it would seem that many of those who had complained about the poor MF AM service were not in a hurry to move to FM. (Although I suspect that the uptake of FM was higher amongst Third Programme listeners than the average, given that it had content that was more demanding in terms of audio quality.) In part though the slow uptake may have been because by the late 1950s a large part of the evening audience had moved from radio to TV, and their daytime MF AM reception was deemed to be adequate.

The use of AM sound for System A television did not really create a precedent for VHF sound broadcasting. The attached excerpt from BBC Monograph #39 more-or-less why AM, although not as good as FM, was acceptable for television.

BBC Monograph #29 p.10.jpg


Some VHF-AM receiving equipment was made available on a normal commercial basis. At least three makers included VHF-AM in the initial FM tuners, namely Lowther, Chapman and Sound Sales. There may have been others. The initial design brief for the Quad FM included VHF-AM capability, but this was not needed by the time that it was in production.

WW 195306 p.48.jpg

WW 195312 p.135.jpg

WW 195406 p.86.jpg


How the above-mentioned tuners handled AM demodulation is unknown. But quite possibly they used the limiter grid, as was done by Jason with its JTV/2 FM-TV Sound Tuner. The latter had a three-stage IF strip, consisting of an EF89 with agc, an EF89 with fixed gain and an EF80 limiter, which fed the FM discriminator. Both the agc and the demodulated AM were taken from the EF80 limiter grid, and the AM output circuit included a diode noise limiter. Thus the IF strip differed from the customary FM-only three-stage type, which usually consisted of a gain stage (perhaps with agc), a combined amplifier/high-level limiter (from whose grid the agc feed was usually taken) and a regular limiter.

The Quad FM (whose core design was done by Geoffrey Horn, evidently in part from a Brimar original circuit) IF strip shows what might be evidence of its original VHF-AM option intent. It originally comprised a pair of 6BH6 gain stages, non-limiting, followed by a 6AU6 limiter, whose grid provided agc bias for the 6BJ6 RF stage. With the second iteration of the Series B version, the RF stage was changed to a 6BH6 without agc, and the 1st IF stage was changed to a 6BJ6 with agc of the “fast” type. That is, effectively, the agc bias included any AM that was present, fed back in antiphase to improve overall AM rejection. Thus the 6AU6 limiter grid was acting as an AM demodulator.

Whilst grid leak AM demodulation has by the conventional wisdom higher distortion than the diode variety, one assumes that Jason et al were able to pick valve operating conditions and time constants, etc., to obtain adequately low-distortion. Maybe the use of sharp cutoff valves helped.

MF AM stereo systems were first proposed in the USA in the late 1950s, at about the same time that FM stereo was under consideration. C-QUAM was a much later development, though. With MF one constraint was that any system chosen must fit into a standard MF channel, ±15 kHz. All proposed systems were variants on theme of an AM envelope carrying the sum signal on a carrier that was somehow phase-modulated with the difference signal. With VHF-AM, the bandwidth constraint would not have been there, thus allowing the use of a subcarrier. Nonetheless, one imagines that C-QUAM would work well.

Perhaps indicative though is that when the time came to add stereo/bilingual sound to TV system L, the French chose NICAM on the same carrier frequency as was used for system B/G/H, thus chopping into the vision bandwidth, which was reduced from 6 to 5.1 MHz as a consequence. I wonder if C-QUAM was considered, given that it was established technology with decoding ICs readily available. But whilst it would have worked for stereo, I doubt that it would have been satisfactory for bilingual sound.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Thu Jul 31, 2014 12:31 am

It may also be noted that VHF-AM broadcasting had been tried in the USA from circa 1937. So-called APEX stations were allocated frequencies either at the high end of the HF band (around 26 MHz) or in the 40 MHz band. The formal arrival of US FM was in 1941, although it had been in use earlier. Possibly some APEX stations were allowed to continue broadcasting for a few years, much as was the case for early “low-band” FM transmitters.

So effectively, the AM vs. FM issue had been reviewed and decided well before the UK considered any form of VHF broadcasting. In that light, BREMA’s request that VHF-AM be considered looks rather disingenuous.

Still, the VHF-AM possibility was recycled in the USA, judging by the attached item.

WW 194808 p.297.jpg
WW 194808 p.298.jpg


That article is not particularly convincing, though. It is allegedly about hi-fi AM, yet early on is this statement: “A compression amplifier is used to maintain a relatively high modulation level”. Not a good start, I should say.

The claim that AM allowed a greater number of channels within a given spectrum is open to doubt, as well. Allowance for likely oscillator drift in domestic VHF receivers using the technology of the period would have required reasonably wide IF passbands, much wider than required for the AM sidebands alone. I am not sure what channel spacing applied to VHF radio-telephones in the late 1940s, but through the 1950s I think 50 kHz was the minimum, with 25-30 kHz and the then 12.5 kHz not appearing until the 1960s once improved techniques allowed higher stability. Not only that, but for broadcasting, sooner or later the impulse noise issue would have reared its head, and to address this with suitable limiters required IF bandwidths commensurate with those for FM.

Even with wideband VHF-AM, the stability issue would to some extent have negated the BREMA argument that AM receivers were simpler than those for FM. Even where the manufacturers made heroic efforts to obtain oscillator stability with their FM receivers, the residual drift required the use of at least mild AFC. Thus the Leak Troughline had AFC from the start. AFC was added to the Quad FM at the Series B iteration. Lowther fitted AFC to its FM Mk II tuner, and offered an AFC retrofit service for its original VHF-AM/FM tuners. Chapman featured AFC at least from its FM91 iteration, if not earlier. From that one may infer that VHF-AM receivers would also have benefitted from AFC, and since AFC requires a discriminator of some sort, once it is added one virtually has an FM-AM receiver. A counter-argument was that whereas FM receivers needed to be tuned to the centre of the IF passband for minimum distortion, that aspect was less critical with AM, although that would depend upon passband shape. A diode demodulator needs to be presented with a symmetrical double sideband signal to minimize distortion, so if an incoming signal were displaced too much to one side of the passband, distortion would rise. Not only that, but noise limiting would be compromised in that any pulses on the side of the carrier closest to the band edge would be stretched, which could put them beyond the reach of the limiter. So it would be desirable to tune the VHF-AM receiver to centre-channel, but that would have been difficult to do without a centre-channel type tuning indicator, and the latter really required a discriminator.

All of that said, I don’t think that the UK setmakers made much use of AFC on their integrated AM-FM and FM-only receivers of the valve era, although I have a notion that Murphy did use AFC (for FM) on some of its TV-FM receivers.

That leads to another thought; had the UK opted for VHF-AM, then I imagine that there would have been TV-AM receivers rather than TV-FM receivers. A TV-AM receiver would have had a simpler sound demodulator section than its TV-FM counterpart, but would still have run into selectivity problems with the usual TV 38.15 MHz sound IF strip. So dual-frequency (10.7 and 38.15 MHz) IF strips or dual-conversion (à la Murphy) would likely still have been required.

The TV-FM receiver was an interesting part of the early FM scene in the UK, and essentially revived a 1940s American idea. The rationale seemed to be that in 1955 and later, many listeners/viewers would need to buy both a new radio receiver to allow access to the FM broadcasts and a new TV receiver to access the new ITV broadcasts and in some areas. Assuming that such listeners/viewers already had a satisfactory AM radio receiver, then acquisition of a combined TV-FM receiver, at a marginal price increment as compared with a TV-only receiver, killed two birds with one stone.

In the USA in the 1940s, both TV and FM were new and initial purchasers could access both by acquiring a combined TV-FM receiver, whilst retaining their existing AM radio receiver. The concept probably suffered a premature death though by the arrival and widespread use of intercarrier sound for TV receivers, whose benefits (to the setmakers) in terms of reduced valve and component counts would have been negated had FM radio reception been included.

The natural life-cycle of TV-FM receivers was probably limited anyway. Within say 5 to 7 years many of the AM radio receivers whose existence was the reason for buying a combined TV-FM receiver would have come up for replacement, and most likely the next purchase would have been an FM-AM receiver. Thus there was no pressing need for the next TV receiver acquired to include FM radio. In the UK case, I suspect that the arrival of dual-standard TV receivers pretty much killed off the TV-FM concept, although it shouldn’t surprise me if sales were already waning. I think that there were one or two dual-standard TV-FM models, but I’d guess that these would have been a bit compromised by using a 6 MHz IF for FM reception.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Thu Jul 31, 2014 12:37 am

colly0410 wrote:I've been yabbering to my mate Jack again, (the one who said the UK should have used a modified system N in the post I put in the TV section) & he said "we (the UK) should have used wide bandwidth AM on VHF band 2." I thought "hmm, not sure I agree with that" but nodded politely. To push home his point he said "remember how good 405 sound was with a good speaker!" & from what I remember it did sound good. I never thought to ask him how VHF AM would have coped with stereo, I wish I had, would the pilot tone sub-carrier system have worked on AM?


It would be interesting to know what Jack-the-lad makes of our collective musings on both this (VHF-AM) and the 625-line issue. Some counter-arguments perhaps? At least though he specifically nominated wideband VHF-AM.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Thu Jul 31, 2014 4:11 am

It has since occurred to me that in the 1970s and through to the end of 405-line TV broadcasting, there was a way to effect a reasonable AM vs. FM comparison with hi-fi equipment, albeit with TV sound.

The Motion Electronics TV Sound Tuner, released in 1971, was apparently switchable between AM and FM. It could also be had with VHF and/or UHF TV channel coverage. So if one had the VHF-UHF version connected to appropriate aerials and to a suitable hi-fi system, it would have been possible to switch between BBC1 or ITV on VHF-AM and BBC1 or ITV on UHF-FM. Of course the comparison could have been clouded by possible transmission path differences. And UHF FM TV sound had ±50 kHz maximum deviation, lower than the ±75 kHz of FM broadcasting, which would have slightly lessened any FM advantage. The differences might not have been all that great with reasonable RF signal strengths. With attenuated the RF inputs, VHF-AM performance would, I think, have dropped off first.

TV Sound Monitor p.01.jpg
TV Sound Monitor p.02.jpg


The Motion Electronics TV tuner was effectively heir to the Jason JTV/2, which had disappeared around 1967. Jason had never updated it to cover the UHF channels though, possibly because by the time UHF came on to the scene, it was fading as a company.

Lowther also introduced a TV sound tuner in 1971, but its model was UHF-FM only. Back around 1960 it had announced a couple of models, including one that looked as if it were intended to receive the BBC experimental stereo broadcasts using FM and TV sound transmitters. But they did not seem to make it into production.

Lowther Catalogue p.10.jpg


But the existence of TV sound tuners with VHF-AM channel coverage does support the original comment:
colly0410 wrote:To push home his point he said "remember how good 405 sound was with a good speaker!" & from what I remember it did sound good.


Of course, in the UK with its preference for television receivers with transformerless power supplies, it was not easy to safely extract a line-level audio feed to a hi-fi system, and even if that was done, the quality might not be all that marvellous. Hence the need for independent TV sound tuners. In the US, different solutions seemed to have been available:

Electronics World 196201 p.12.jpg


Motion Electronics went on to produce a conventional TV tuner (providing baseband video and audio outputs) in the mid-1980s. By then, such units had acquired domestic status by virtue of the introduction of “Component TV” circa 1981, by Sony and others.

100 Ch TVSM p.01.jpg


Actually, back in the 1970s I had thought that a better way – simpler to operate - to integrate TV sound into a hi-fi system would be to use an industrial tuner, such as the Sony TU-1100 (which was presentable enough for domestic use) with its audio output fed to the hi-fi system and its video to a monitor. But I am not sure about the sound quality from such a tuner; perhaps it was intended to be more utilitarian than hi-fi.

Sony TU-1100.jpg


That is a digression though. Back to VHF-AM, had the UK chosen that pathway, then likely it would have done so alone. Against the backdrop where the rest of the world was opting for VHF-FM, there surely would need to have been a very compelling case (which there was not) for VHF-AM to have justified its selection.

Cheers,

Steve

 
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Re: VHF AM?

Post by colly0410 » Thu Jul 31, 2014 11:22 am

Some very interesting replies, thanks to all..... Jack has an old computer sat in the corner but I've never seen it on, his eyes are not too good (he's in his late 80's) so perhaps struggles with it, I'm friends with his niece on FB so I'll ask her if he goes online, if he does I'll try & point him here. I meant to talk to him about the 625/system N thing when I saw him but I forgot, I'm rather useless aren't I ? lol..

 
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Re: VHF AM?

Post by colly0410 » Fri Aug 01, 2014 4:53 pm

I've been thinking, (yes I know it'll wear my brain out) would using pre-emphasis on VHF AM reduce noise & impulse interference like it does on FM? I think I remember reading somewhere that Belgium used it on their TV systems that used AM sound, or did I dream that...

 
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Re: VHF AM?

Post by Synchrodyne » Sat Aug 02, 2014 1:11 am

It was not a dream; Belgian TV Systems C (625/50) and F (819/50) both had AM sound with 50 µs pre-emphasis. However, British System A and French Systems E and L had AM sound without pre-emphasis.

In the solid state era, there were TV AM sound ICs that had switchable de-emphasis:

TDA2543.jpg


Pre-emphasis is also used in MF AM broadcasting.

In Japan the NHK uses 100 µs. This may date from the 1980s; this link may well point to a learned paper on the topic, albeit in Japanese: http://ci.nii.ac.jp/vol_issue/nels/AN00 ... 76_en.html.

In the USA, AM pre-emphasis was standardized at 75µs in the mid-1990s, as part of the NRSC standard. This link points to some of the development considerations: http://www.nrscstandards.org/Reports/NRSC-R10.pdf.

Part of the NRSC standard was audio bandwidth limitation to 10 kHz, which evidently did not go down too well with some broadcasters:

Audio 199211 p.22.jpg


In Australia, ABC uses the NRSC 75 µs number with a shelf at 8.7 kHz. Audio bandwidth is at least 7.5 kHz, and at best 9.7 kHz because of the need to cutoff at 10 kHz.

The FM and AM situations are somewhat different, though. The triangular noise spectrum of FM almost demands that pre-emphasis and de-emphasis be used, whereas that is not the case for AM. Thus the benefits of using pre-emphasis with AM are smaller, at least in respect of wideband noise. Tibbs and Johnstone provide a good treatment of the topic:

Tibbs & Johnstone fp.iii.jpg
Tibbs & Johnstone p.96,97.jpg
Tibbs & Johnstone p.98,99.jpg
Tibbs & Johnstone p.100,101.jpg
Tibbs & Johnstone p.102,103.jpg


It would appear that a primary reason for using pre-emphasis with AM would be to reduce the effects of interference more than noise. In the TV case there may have been concerns about fH getting into the AF path. With impulsive interference and wideband AM, I think that it would be better to place the de-emphasis after the noise limiter, otherwise the noise pulses would be stretched and perhaps be not differentiable. De-emphasis by itself might not be all that effective at impulse noise reduction. In the MF AM case, another reason for the introduction of pre-emphasis was the fact that so many receivers were poorly engineered and as a result had AF roll off at relatively low frequencies, sometimes from even below the voice bandwidth upper limit of 3.4 kHz.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Sat Aug 02, 2014 2:49 am

Keller also provided a good, concise treatment of the triangular (FM) vs. square (AM) noise issue.

Keller fp.iii.jpg
Keller p.26,27.jpg
Keller p.28,29.jpg
Keller p.30,31.jpg


Keller also mentions the possibility of constant rate (e.g. 6dB/8ve) pre-emphasis. In fact phase modulation (PM) automatically provides this. As far as I know, PM is often used in NBFM R/T systems. From a receiver viewpoint, PM is sometimes treated as requiring a 750 µs de-emphasis curve. As this provides a 212 Hz turnover point, it is close to delivering a – 6dB/8ve slope over the voice bandwidth of 300 to 3400 Hz. I am not sure that this always applied, though. In the early days when FM R/T was typically ±30 kHz deviation, conventional pre-emphasis may have been used.

As noted by Tibbs and Johnstone, FM broadcasting in the USA started with 100 µs pre-emphasis, then dropped back to 75 µs, although exactly when the change was made I do not know. Possibly it was when the change was made from the 42 to 50 MHz to the 88 to 108 MHz band. US TV sound also changed from ±75 kHz deviation with 100 µs pre-emphasis to ±25 kHz deviation with 75 µs pre-emphasis. Whether deviation and pre-emphasis changes were coincident, and when they happened I do not know. The 50 µs number used in Europe may have originated from the Russian work on 625-line TV, from whence also probably came the ±50 kHz deviation number used in European TV sound (and OIRT Band I FM broadcasting). In the 1970s or thereabouts, 25 µs was also proposed – and used experimentally – in conjunction with Dolby B noise reduction.

For AM, it looks as though the 50, 100 and 75 µs numbers, in that chronological order, have been/are being used. It is interesting to note though that back in 1941, NTSC (1st) assumed that 100 µs pre-emphasis regardless as to whether AM or FM was chosen for the TV sound channel. As far as I know, the BBC VHF-AM experiments did not use any pre-emphasis.

It would be good to find schematics for the previously mentioned tuners that had VHF-AM capability, namely the Lowther AM-FM, the Sound Sales and the Chapman FM81 (1st iteration; the 2nd iteration was FM-only, as was the later Mk II). But I’d guess that the chances are minimal.

Cheers,

Steve

 
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Re: VHF AM?

Post by colly0410 » Sat Aug 02, 2014 10:37 pm

Some very interesting reading there Steve, thanks. I found the part about the difference between phase & frequency modulation informative as it's a thing I'd never thought about before....

I expect if we had adopted VHF AM the transmitter electricity bills would have been higher. I don't suppose any non UK manufacturers would have bothered making VHF AM sets so no competition so set prices would be higher... I've got one of those FM modulators so I can play my downloads/you tube ect through my hi-fi, that would be as much use as a chocolate gas stove on VHF AM, unless it used slope detection.. :)

 
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Re: VHF AM?

Post by Michael Watterson » Sun Aug 03, 2014 7:50 pm

The 50s VHF-AM Mobile gear was all 50KHz channels. To achieve it, all rigs used two sets of crystals, one for TX and one for RX with multipliers. No free running L.O. on 50KHz channels.

The last non-crystal VHF mobile was probably WWII era and before the 1950s (like option B set on WS 19). The WS No.88 (1947) was Band I and used crystals. But already was FM, not AM, I think.
http://www.wftw.nl/wireless88.html
http://www.radiomuseum.org/r/mil_gb_wir ... _no88.html
Many commercial VHF AM mobile was derived from military designs. But FM was quickly the mode of choice except for aircraft where it is useful to NOT have the FM capture effect.

 
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Re: VHF AM?

Post by Synchrodyne » Thu Aug 07, 2014 1:36 am

In the late 1930s the FCC planned on using 40 kHz channels in the 40 MHz band for APEX stations. With variably tuned domestic-quality receivers, I somehow doubt that that plan would have been very successful though. And not only that, once the impulse noise problem came into view, there would not have been enough receiver IF bandwidth to deal with it effectively, assuming that design was based upon providing a reasonable level of adjacent channel rejection.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Mon Aug 18, 2014 10:53 am

I was surprised to discover that Germany, which initially took the lead with VHF FM broadcasting in Europe, also did consider VHF AM as one of its options, as recorded here:

WW 195204 p.141 German FM.jpg
WW 195204 p.142 German FM.jpg


Also, at the 1952 ITU Stockholm VHF allocation meeting, Belgium as well as the UK reserved the right to use VHF AM, as noted here:

WW 195210 p.433 ITU Stockholm VHF AM or FM.jpg


The Belgian position is not so surprising in retrospect. Earlier in 1952, it had decided to adopt AM sound for its variants of the 625- and 819-line TV transmission systems. So using AM for VHF Band II sound transmission would have been consistent with that decision. Presumably though the weight of evidence in favour of FM and the need to fit into a European Band II plan put paid to actually using AM.

Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Mon Aug 18, 2014 11:02 am

Evidently there was a receiver designed and built especiallly for VHF FM and VHF AM comparison purposes:

WW 195201 p.35 AM-FM Comparator.jpg
WW 195201 p.36 AM-FM Comparator.jpg


Cheers,

Steve

 
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Re: VHF AM?

Post by Synchrodyne » Tue Aug 19, 2014 12:09 am

And here is the Wireless World précis of the BBC report on the FM vs. AM issue.

All things considered, I think that it would have been difficult to make a cogent case for VHF AM.

Cheers,

Steve

WW 195201 p.10 VHF Broadcasting.jpg

WW 195201 p.11 VHF Broadcasting.jpg

 
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Re: VHF AM?

Post by colly0410 » Tue Aug 19, 2014 5:30 pm

It seems that VHF AM was an also ran in the the race & VHF FM won by a a few furlongs, never mind VHF AM at least you tried. Congratulations VHF FM for winning the race & still going strong all these years later, although the digital hooligans are snapping at your rear & wanting to take over the lead..

 
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Re: VHF AM?

Post by crackle » Tue Aug 19, 2014 10:03 pm

How is the digital information of say DAB and freeview relayed over the airwaves, AM, FM, neither?
Thanks
Mike

 
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Re: VHF AM?

Post by Terrykc » Tue Aug 19, 2014 10:40 pm

A combination of AM and phase modulation - QAM - on a large number of sub-channels, Mike.

The 8MHz channel is split into 6,817 sub-channels, with the carriers only 1170Hz apart. The HD channels use even more - 27,841, a mere 280Hz apart!

The modulation is similar to that used for the colour subcarrier in analogue TV - quadrature amplitude modulation (QAM) - except the amplitude and phase components are in fixed steps, rather than continuously varying, as in an analogue system.

You might find the explanation here: http://www.ukfree.tv/fullstory.php?storyid=1107051920 informative ...

 
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Re: VHF AM?

Post by Michael Watterson » Wed Aug 20, 2014 3:46 pm

COFDM and COFDMA are strictly speaking not modulation schemes but spectrum utilisation schemes as each carrier can be PSK, APSK, QAM, AM, FM etc. It's the principle of transmitting the information in parallel at a much lower speed than a single serial connection.

i.e. 6700 "wires" or channels with slowly changing information vs 1 wire with fast changing information. You could make a COFDM system with several thousand 300 baud modems and SSB on suitably spaced channels.

Shannon / Nyquist means the over all power for the transmission and bandwdidth for same SNR is the same. Except there are interesting aspects:
1) The guard band at either side of the COFDM signal is much smaller, so with practical filters you can either pack the overall channels tighter or have data closer to the edge.
2) The Forward Error Correction and data is spread in time and individual channel so a fixed narrowband interference etc interferes with less data. Better overall SNR for non-random interference.
3) It's possible to decode the same signal arriving at different times (ghosts, multipath), up to a maximum time and improve the reception. With a single carrier the same amount of multipath would be worse.

None of these aspects are an advantage for Cable or Satellite. So only Terrestrial transmission uses the extra complexity of COFDM. The COFDMA is used in certain 4G systems where the carriers are dynamically assigned to different connections rather than the TDMA, CDMA or Data-multiplexing (DOCSIS downlink) schemes. Multichannel DAB DTT etc multiplexes separate slower data streams for each program channel, then adds the FEC and finally modulates all the carriers with the single high speed serial stream split in to 1/6700 th speed streams if approximately 6700 carriers etc (8192 mode).

A disadvantage of CODFM and COFDMA is that very much more linear Power Amplifiers on Transmitter and Preamp etc on receiver is needed or the noise margin is degraded.

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