It is currently Sun Nov 19, 2017 1:15 pm

VHF AM?

Domestic Valve & Transistor receivers, Radiograms, Gramophones, Amplifiers, Hi-fi, Speakers, Record players, Music centres, Tape machines, Cassette players & Jukeboxes
 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Tue Aug 26, 2014 5:16 am

Looking again at the original question, I suspect that the use of pilot-tone stereo with VHF-AM could have problematical from the impulse noise reduction viewpoint. The French bilingual TV example was, as far as I know, in the Band III channels where the impulse noise problem is less than that at Band II.

The effectiveness of impulse noise suppression would appear to depend upon both the IF bandwidth, namely the wider the better (less pulse stretching) and the effective “crossover” point between audio and noise, a lower number here providing greater suppression. With a stereo subcarrier, the crossover point – assuming that noise suppression was done before decoding - would need to be a little above 53 kHz, whereas with a mono signal, a little above 15 kHz would suffice. In the stereo case, noise in the subcarrier frequency range would thus be troublesome.

Some of this is brought out in the attached item from WW 1951 October. It was also one of the more enlightened contributions on the topic, which as might be imagined, generated quite a lot of correspondence.

WW 195110 p.407 VHF-AM.jpg


Thus had VHF-AM been the choice, I suspect that when the time came to add stereo, there would also have been a hard look taken at the systems that had been proposed for MF AM in the late 1950s and early 1960s, all of which were designed to fit within a 30 kHz channel. These included AM-FM (using NBFM for the difference channel) and I think by then Kahn was already working on a compatible independent sideband approach. C-QUAM was still in the future but probably it could have been invented “earlier” had it been needed. The Kahn approach to obtaining compatibility with independent sidebands looked to be somewhat complex, and it has been opined that better would have been to apply the C-QUAM approach to obtaining compatibility, bearing in mind that ISB itself is phase-shifted form of QUAM. Decoding of any of these systems using valve technology might though have involved more complexity than decoding the pilot-tone system. So the choice of VHF-AM stereo system would have had the makings of an interesting debate.

VHF-AM receiving equipment mentioned to date had been of the dual AM-FM type. That duality was logical both because the raison d’être for the Wrotham experimental transmissions was to compare the two modulation types, and because the makers no doubt anticipated that regardless of the UK outcome, they would be making VHF-FM receivers for export. Nevertheless, it appears that Lowther did offer a VHF-AM only tuner, and that this was earlier than its VHF AM-FM model.

WW 195207 p.257 Lowther Tuners.jpg


Cheers,

Steve

 
Posts: 7156
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: VHF AM?

Post by Michael Watterson » Thu Aug 28, 2014 5:37 pm

Bi-lingual or dual sound track needs two real channels. The cross talk with any L+R and L-R matrix system is too high no matter how the L-R is carried. This no doubt is why Germany and others used two separate FM carriers.
On old Analogue Microwave FM links with 6 FM audio carriers added to the video baseband prior to modulation the last two channels were too poor S/N for broadcast material. Often 3 would carry radio distribution and one the audio for the video.

Analogue Satellite TV used exactly the same scheme (though not the same audio channel spacings), but I forget what the maximum number of channels were. The 3rd & 4th often unrelated Radio and prone to vision induced noise on poorer satellite receivers, or if mistuned or if AFC was poor.

The 2.4 GHz and 5.8GHz Video Senders, even if marketed as "Digisenders" all use 6MHz and 6.5MHz FM carriers for audio. These are simply added to the baseband video. So like the old BBC Analogue Terrestrial Microwave and Analogue Satellite, the two channels have best SNR if you use no video. The L & R are completely separate.

Any matrix approach only works for material intended to be binaural or stereo.

For the USA market Grundig had a single pentode decoder as the USA had Multiplex Stereo before Germany, who preceded UK.
It used two diodes as mixer to decode the 38KHz DSBSC and simple passive L+R & L-R matrix. The Pentode was cathode fed with the 19kHz and acted as a frequency doubler with Anode 38KHz driving diode mixer.

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Fri Aug 29, 2014 5:12 am

The French bilingual TV sound system used in Algeria in the 1950s used a single AM carrier with time division multiplex, at least judging by the information that I have come by. How well it worked, and what the crosstalk was like is unknown, though. The switching was slaved to the line timebase, so assuming that receivers had a decent line timebase with AFC, then it was tantamount to PLL decoding. And the latter did not arrive for FM MPX until 1970-71, with the Portus and Hayward discrete decoder and then the CA3090, µA758, etc., ICs. Although even back in valve days, there were locked-oscillator decoders, which might be seen as a rudimentary form of the synchronous type.

More generally, two distinct channels, as in Zweiton, are apparently not always a pre-requisite for bilingual or otherwise separate program channels. FM subcarriers were used for this purpose. In the USA, SCA used FM subcarriers from the early 1950s. Initially 41 kHz was common, moving to 67 kHz after the advent of stereo with 92 kHz later added as a second SCA channel. The Japanese FM-FM TV sound system had a 2fH FM subcarrier that could be used either for the stereo difference channel or a second audio program. A corresponding system was developed for European Systems B/G/H, but it was found that the German Zweiton (IRT) two-carrier system was a bit better in mountainous regions where multipath was prevalent. The American MTS (Zenith-DBX) TV sound system had a 2fH AM subcarrier, DBX-compressed, for the stereo difference channel, and a 5fH FM subcarrier, also DBX-compressed, for the second audio program. A 2nd FM subcarrier, at 6.5 fH, was available for non-broadcast professional purposes.

Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Thu Sep 04, 2014 11:26 pm

In terms of VHF-AM receiving equipment, evidently Lowther offered a pre-tuned VHF AM-only unit, and this may have been made available slightly ahead of its variably tuned VHF AM-FM unit.

WW 195207 p.257 Lowther Tuners.jpg


Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Wed Dec 10, 2014 2:20 am

An interesting aspect to this is that before the VHF-AM option reared its head to cause several years of chaos, confusion, delay and obfuscation, some of the setmakers had actually offered VHF-FM receiving equipment. Perhaps they, like the BBC, had figured out that FM was the logical choice, and so had anticipated introduction of an FM service in the UK.

HMV and Kolster Brandes had both shown FM receivers at RadioOlympia 1949, as recorded in WW 1949 November.

WW 194911 p.434 FM Receivers.jpg
WW 194911 p.435 FM Receivers.jpg


And WW 1950 January included an item about a new English Electric TV-FM receiver.

WW 195001 p.20 English Electric TV-FM.jpg


Meanwhile, as the issue was debated in the UK, Germany and Italy, both also with a shortage of MF channels, simply moved ahead with installing FM networks, as reported in WW 1949 November.

WW 194911 p.445 European FM.jpg


Cheers,

Steve

 
Posts: 7156
Joined: Sun Aug 14, 2011 11:02 am
Location: Co. Limerick

Re: VHF AM?

Post by Michael Watterson » Wed Dec 10, 2014 8:58 pm

KB = ITT = Schaub-Lorenze after WWII. VHF-FM in Germany in 1949
The Allies pinched the German AM frequencies, so that was a driving factor

HMV = EMI (I think active in Italy)
I wonder were some Italian AM used for AFN etc?

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Mon Dec 22, 2014 3:33 am

An American perspective on the UK VHF-AM vs. VHF-FM debate was provided in an article in Radio-Electronics (R-E) for 1951 October, see: http://tinyurl.com/RE-195110, pp.51, 52.)

The thought expressed therein that the outcome would be free of political or commercial bias was, I think, rather optimistic. The technical answer was known before the comparative trials were considered. And I understand that it was BREMA who pressed for the field comparison, in turn pressed by Stanley of Pye, who was antagonistic to any kind of VHF sound broadcasting and if it had to be, would have chosen the minimalist narrow-band AM option (which precluded the use of impulse noise limiters). So it was commercial influence that led to the trials.

The R-E article provided a somewhat more detailed description of the Fitton (Ambassador) comparative receiver than I have seen elsewhere. It looks to have been quite a tour-de-force. I wonder if any have survived; it could be a collector’s piece by now. It had a 14 MHz IF, which was non-standard for FM receivers. I cannot trace this directly to TV sound IF practice of the time, either; 14.5 MHz seems to be the closest approach there. The FM 1st limiter acted as AM demodulator, meaning that it was effectively of the grid-leak type where the AM signal was concerned. In view of the rudimentary comments that are usually made about the (non-)linearity of the grid-leak demodulator, at first glance that seems strange. But on the other hand it is simply using the cathode and grid of a pentode as a diode, so with the appropriate attention to detail, it should behave much like a diode demodulator. And (carefully specified) diode circuits were used for hi-fi AM tuners back in the valve era, when fundamentally better types were beyond easy realization. Anyway, it seems unlikely, given the overall nature of the comparator receiver, that anything less than immaculate demodulator performance (by the standards of the day) would have been accepted.

The same technique, that is the use of an FM limiter grid as an AM demodulator, appeared again in the Jason JTV FM-TV sound tuner circa 1958 (and was carried over to the JTV2 and Mercury II/Monitor models.) Also, there is some evidence that the prototype Quad FM might have incorporated the same arrangement to facilitate VHF-AM as well as VHF-FM reception, although it was not used on the (1955) production version, which naturally by that time was FM-only. How the other makers of VHF-AM/FM tuners (Chapman, Lowther, Sound Sales) accomplished AM demodulation is unknown, and I suspect that there is Buckley’s chance of finding out. But...they were in uncharted waters, and the Fitton/BBC precedent may have been quite persuasive.

Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Fri Jun 26, 2015 3:50 am

I recently read an article in Wireless World for 1946 October outlining the BBC’s initial test program on VHF-FM and VHF-AM transmissions. Unsurprisingly, the results came out in favour of FM. Probably a decision in favour of FM could have been made – and one might also say should have been made – at that time. The trials and deliberations that followed can be seen as having been superfluous, but were forced upon the BBC by other parties, amongst whom were those who were antithetical to any form of VHF sound broadcasting.

The 1946 October issue of WW may be found at: http://www.americanradiohistory.com/Wir ... gazine.htm.

Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Sun Jul 12, 2015 4:41 am

There was also a similar commentary on the BBC’s initial VHF FM and AM comparative work in the American magazine Radio Craft for 1946 October. I have attached a copy, and the original may be found at: http://www.americanradiohistory.com/Rad ... _Guide.htm.

Recorded therein is that the BBC looked at pre-emphasis numbers of 25, 50, 75 and 100 microseconds, narrowing this down to a choice between 50 and 75 microseconds, of which 50 microseconds was deemed to be the better of the two.

In the USA, FM started with 100 microseconds pre-emphasis, but this was changed to 75 microseconds. The change might have been coincident with the move from the 42-to-50 to the 88-to-108 MHz band, announced by the FCC mid-1945. There is indirect evidence at least that the 100 microsecond standard was still in force at about the time that the band shift was announced.

Cheers,

Steve
Attachments
Radio Craft 194610 p.29.tif

 
Posts: 249
Joined: Sun Jan 19, 2014 8:09 pm
Location: Hucknall, Notts.

Re: VHF AM?

Post by colly0410 » Tue Sep 22, 2015 11:50 pm

I think I read somewhere that the receiver bandwidth on the 405 line AM sound system was fairly wide to allow for frequency drift & this is why the sound was excellent quality. Did the French systems E & L & Belgian systems C & F sound have the same wide bandwidth?

 
Posts: 20
Joined: Wed Oct 01, 2014 1:48 am

Re: VHF AM?

Post by dtvmcdonald » Wed Sep 23, 2015 4:33 am

By accident I was listening to my 1942 Pilot 300 FM radio (USA) when I came across this thread.
This was recently repaired and aligned by me. No capacitors needed replacing.

It was a 42-50 MHz model. The oscillator was below the signal frequency. This means that
it tuned the lower part of the current FM band on the oscillator second harmonic, which is extremely
large. I easily modified just the antenna tuned circuit to tune the current band. It has no RF stage and a 14Q7 pentagrid converter, using the first screen as a RF-grounded plate. Separate triode oscillator.
Sensitivity is abysmal, but it sounds great (I listen to only classical music). Also, warmup drift is terrible.

The question is ... what was the de-emphasis. I looked at numerous schematics of 1941-1944
(42-50MHz) sets and the de-emphasis networks are all nominally either 50 or 68 usec.

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Wed Sep 23, 2015 7:37 am

dtvmcdonald wrote:The question is ... what was the de-emphasis. I looked at numerous schematics of 1941-1944 (42-50MHz) sets and the de-emphasis networks are all nominally either 50 or 68 usec.


It should be 100 microseconds.

The parameters for the original US FM service in the 40 MHz-band were ±75 kHz deviation and 100 µs pre-emphasis.

As far as I know, the change to 75 µs pre-emphasis occurred with the change to the 88-to-108 MHz band, but I have not seen absolute proof. The circumstantial evidence is very strong, though.

Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Wed Sep 23, 2015 7:50 am

colly0410 wrote:I think I read somewhere that the receiver bandwidth on the 405 line AM sound system was fairly wide to allow for frequency drift & this is why the sound was excellent quality. Did the French systems E & L & Belgian systems C & F sound have the same wide bandwidth?


Most likely they did. Another reason for the wide bandwidth was to enable the use of impulse noise limiters, which required preservation of noise pulse rise-time and hence wide bandwidth.

With typical two-stage AM sound IF strips and conventional IFTs with sane Q numbers that did not make for difficult factory alignment, the “natural” bandwidth was probably enough to accommodate oscillator drift and the noise limiter needs.

I have seen bandwidth numbers for 39.2 MHz sound channel SAW filters for French System L of both 400 and 600 kHz at the -6 dB points. I imagine that by the time SAW filters arrived, most System L receivers used AFT, so accommodating oscillator drift would have been less of an issue. In fact fairly precise tuning an maintenance thereof would have been desirable for the proper functioning of those (not all) quasi-synchronous sound demodulators that included tank circuits, as for example in the case of the TDA2543 IC.

Cheers,

Steve

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Wed Sep 23, 2015 8:10 am

Synchrodyne wrote:As far as I know, the change to 75 µs pre-emphasis occurred with the change to the 88-to-108 MHz band, but I have not seen absolute proof. The circumstantial evidence is very strong, though.


Also, that meant that dual-band FM receivers, of which I think a few were produced, should have had switchable de-emphasis, 100 µs for the 40 MHz band and 75 µs for the 100 MHz band. Whether they did or not is open to question. I suspect that some setmakers took liberties, for example, reducing de-emphasis to in effect provide a treble boost to offset natural treble cut in the amplifier and/or speaker. Plus I think that circuit stray capacitance was sometimes included in de-emphasis capacitor calculations.

Way back when I came across a valve-era NZ monochrome TV receiver that had no de-emphasis circuit as such, but rather a regular radio receiver-type “tone” control of the treble cut type that probably provided nearly approximately the correct 50 µs de-emphasis at somewhere in the middle of its range. This was not apparent though until I arranged an audio feed (from the across the volume control) to a hi-fi system. (The TV receiver at interest had a proper, isolated power supply, none of that nasty AC-DC stuff). Then the sound was annoyingly very overbright, which led to a closer look at the TV circuitry.

Cheers,

Steve

 
Posts: 249
Joined: Sun Jan 19, 2014 8:09 pm
Location: Hucknall, Notts.

Re: VHF AM?

Post by colly0410 » Wed May 25, 2016 8:11 pm

If we (or some other Country) had used VHF-AM & we wanted to use stereo, would it have been better to use a a 38 khz sub-carrier system as used on FM, or C-QUAM as used on medium wave stereo TX's?

 
Posts: 316
Joined: Thu Jun 14, 2012 4:04 am
Location: Mt. Maunganui, New Zealand

Re: VHF AM?

Post by Synchrodyne » Thu May 26, 2016 2:37 am

My guess is that both would work, but that both would have problems.

We know that C-QUAM works and works quite well for MF ground wave reception. At VHF, the channel widths are such that there would be less concern about curtailing the PM sidebands, and it might be possible to extend the difference channel to a full 15 kHz bandwidth, rather than the 7.5 kHz used at MF.

So far, so good. But C-QUAM is probably susceptible to multipath reception. The C-QUAM decoder ICs, such as the MC13020, include a co-channel carrier detector that inhibits stereo operation when co-channel interference is above a specific low level. With MF reception, this is more likely to happen outside of the primary reception zone, where there is more likely to be interference from another skywave or the desired transmitter’s own skywave. I suspect, but I don’t know for sure, that at VHF, multipath would be seen by the decoder as co-channel interference and inhibit stereo operation. If so, as multipath could occur anywhere in the primary service area, VHF C-QUAM might suffer from a higher incidence, possibly or probably an unacceptably higher incidence, of switching to mono than would MF C-QUAM in its ground wave primary service area.

Noise limiters were regarded as highly desirable, if not essential for VHF AM, and I think that most UK 405-line TV receivers incorporated such. The conventional diode type that cutoff on fast rise-time pulses could also be thought of as a frequency division device, and required an input bandwidth of around 100 kHz or so. It may be recalled that the UK VHF modulation argument was to some extent three-way, namely FM, narrowband AM and wideband AM, the last-mentioned enabling the use of receiver noise limiters. So for VHF C-QUAM, decoders would have required wide “audio” bandwidths. That should not have been too difficult; one may easily imagine a 10.7 MHz, wideband version of the MC13020. Perhaps IC-based noise limiters would have been used, as well. I think that FM stereo noise blankers such as the Philips TDA1001 and Toko KB4423 would have worked; these could also have been used for AM TV sound although I doubt that they were.

Something else to ponder though is that C-QUAM was developed in the integrated circuit age, and so included decoding features, such as pilot tone cycle counts, that were easily implemented in IC form, but which would have been more difficult to do, at least for economical mass production, in discrete solid sate or valve form. That is not to so that the basic decoding could not have been done well in the pre-IC age. With valve technology, for example one might look for inspiration in the circuitry of the GE YRS-1 SSB adaptor of the late 1940s, which used the Norgaard phasing technique for sideband separation, and the Hazeltine DC quadricorrelator of the 1950s, developed for NTSC colour TV subcarrier recovery and decoding control.

Nevertheless, stereo AM systems for MF were developed in the late 1950s and early 1960s, and these were of course intended to work with relatively simple valve decoders. RCA did a lot of work in this field, and concluded that with the technology of the time, an AM-FM system, with the difference channel frequency modulated on to the AM carrier, was the best choice. This is recorded in detail in RCA Review for 1960 September, p.299ff, and available here: http://www.americanradiohistory.com/RCA ... ue_Key.htm. The BBC made brief mention of the MF stereo AM proposals of the period in its Monograph #27 of 1960 April, “A Summary of the Present Position of Stereophonic Broadcasting”, available here: http://downloads.bbc.co.uk/rd/pubs/arch ... aph_29.pdf.

Had VHF-AM been a “going concern” in the UK back when FM stereo was being debated in the USA, then I imagine that the proposed MF AM stereo systems would have been the starting point for the development of a VHF AM stereo system. I think that MF AM stereo as well as FM stereo was within the original ambit of the US NSRC (National Stereo Radio Committee), which advised the FCC, but was dropped off, perhaps because of lack of any real commercial interest at the time.

That an AM-AM subcarrier system could work is evidenced by the French bilingual TV sound transmission system of the later 1950s. What works for bilingual sound has a good chance of working for stereo, although the converse is not necessarily true. It was conceived as a TDM system, but a TDM system that transmits just the first set of sampling frequency sidebands is the same as an FDM system whose subcarrier frequency is the same as the TDM sampling frequency. In this case the sampling frequency/subcarrier frequency was 20 475 Hz, the line frequency of the 819-line TV system. And the audio bandwidth was 9 kHz. There was no pilot tone though, as the line frequency reference was obtained from the line timebase (or from separated sync from the demodulated video signal). The availability of an “external” reference allowed very simple TDM decoders. One of the sound IF stages was simply switched at on and off at line frequency, with one or other channel chosen by reversing the relative mark-space positioning. I imagine that a 9 kHz low-pass filter in the audio channel may have been desirable. Possibly a diode noise limiter could have been used ahead of this, although whether subcarrier artefacts would have gotten in the way, so as to speak, I don’t know.

For non-TV applications though, a pilot tone (or residual subcarrier) would have been needed. As this could be recovered only after demodulation, then the simple pre-demodulation decoding system described above could not be used. Instead, normal pilot-tone decoding would be required. Then I think that noise-limiting would have become a problem, at least in the pre-IC age. Noise-blanking circuitry inserted between the demodulator and decoder that also provided for pilot-tone continuity might have required a non-trivial valve count, but I haven’t really thought that one through. It might have helped to keep the subcarrier frequency as low as possible, on the assumption that the effect of impulsive noise increased somewhat with frequency. In that case the FM stereo polar modulation system, with 32 Hz subcarrier, only partially suppressed, might have been a better choice than the Zenith-GE pilot tone system.

Probably some attention would have had to be paid to AM diode demodulator performance, to minimize non-linearities that resulted in cross-modulation and intermodulation involving the subcarrier. In fact it seems likely that the better class VHF-AM receivers, had they existed, would have had AFC obtained from a limiter-discriminator combination, in which case the limiter grid could well have been used for AM demodulation. That this was satisfactory, despite the conventional wisdom about grid-leak demodulation, was evidenced by the fact that it was used in the Ambassador-BBC VHF-AM/FM comparator receiver, which was designed for high-quality reception.

So, no easy answer, but I think we can say with reasonable certainty that had VHF-AM stereo been required circa 1960, then both subcarrier and AM-PM systems would have been considered, AM-PM covering both the QUAM and AM-FM varieties.

Cheers,

Steve

 
Posts: 249
Joined: Sun Jan 19, 2014 8:09 pm
Location: Hucknall, Notts.

Re: VHF AM?

Post by colly0410 » Thu May 26, 2016 10:26 am

Thanks for that brilliant reply..... I love speculating that if so-&-so had happened how would we have done this & that? I.E. If transistors had been discovered first would we have bothered with valves for low power gubbins? I know we can never have a full answer as you can't change history, worse luck. :)

Previous

Return to Radio & Audio



Who is online

Users browsing this forum: No registered users and 3 guests