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Old 4th Sep 2018, 12:42 pm   #21
John10b
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Default Re: Historic AM audio quality

Very informative and interesting Barry, I just wonder why 6db and not 3db is quoted?
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Old 4th Sep 2018, 12:52 pm   #22
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Default Re: Historic AM audio quality

Don't forget that VHF TV sound was AM and so comes within the scope of this thread! And that sound quality was very good, with no treble bandwidth cut. Main issue was ignition interference.
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Old 4th Sep 2018, 1:13 pm   #23
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Default Re: Historic AM audio quality

Unfortunately virtually no TV set took advantage of it but yes the quality of transmission was excellent.
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Old 4th Sep 2018, 1:33 pm   #24
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Default Re: Historic AM audio quality

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Very informative and interesting Barry, I just wonder why 6db and not 3db is quoted?
Cheers
John
I guess that the 3dB 'half-power point' criterion is rather academic and may be barely perceptible, but 6dB down is definitely noticeable.

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Old 4th Sep 2018, 3:02 pm   #25
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Default Re: Historic AM audio quality

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Originally Posted by kalee20 View Post
Don't forget that VHF TV sound was AM and so comes within the scope of this thread! And that sound quality was very good, with no treble bandwidth cut. Main issue was ignition interference.
The BBC were seriously considering using wideband AM rather than FM on VHF in the 40s. They got as far as carrying out some test transmissions.
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Old 4th Sep 2018, 3:41 pm   #26
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The early BBC hand/year books indicate 10kHz at least, given there where no deliberate audio filters it all depended on the audio chain, 'phone lines (long ones) being the worst.

Shortwave today has 5kHz spacing and +/- 5kHz sidebands, if it were like MW today that would be 2.5kHz sidebands.
 
Old 4th Sep 2018, 9:41 pm   #27
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Default Re: Historic AM audio quality

This makes for fascinating reading & perhaps goes some way to explain why an unmodified DAC90A sounds muddy these days.

Source & transmission not being what they once were. Not a major problem for up-market sets with tone controls. That said, my Philco 333 & 444 have a very pleasing tone, being restored, but unmodified.

As for TV sound, I don't think it's ever been fully utilised, NICAM being a later example.

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Old 4th Sep 2018, 10:21 pm   #28
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Default Re: Historic AM audio quality

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Unfortunately virtually no TV set took advantage of it but yes the quality of transmission was excellent.
One advantage of "wideband" AM as used for old TV audio [and video] is that the various simplistic noise-limiters/clippers of the era intended to work against impulse-interference [motor commutators, car ignitions, trolleybus pantographs etc] performed much better when the impulse-noise hadn't been passed through a series of the sort of narrowband IF-stages you'd find in a typical SW/MW/LW broadcast-radio.

Impulse-noise can be considered to have a massively-wide bandwidth, and when passed through narrowband IF stages near or at saturation the impulses undergo what can be imagined as a "bandwidth-clipping" effect which generates harmonics. Wide-bandwidth stages mean that these 'harmonics' are well outside the audible range.
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Old 4th Sep 2018, 11:31 pm   #29
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Default Re: Historic AM audio quality

Regarding impulse noise, IIRC, the circuits variously called ASU, IAC, ANSS, etc., by various manufacturers back in the 70s & 80s, and fitted to their car radios & stereos, were designed to minimise that type of interference to FM radio reception. They worked by taking a portion of the signal, inverting it, and feeding it back, so that a positive noise burst was cancelled by being 'covered' with a negative pulse of the same amplitude & duration.
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Old 4th Sep 2018, 11:49 pm   #30
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Default Re: Historic AM audio quality

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Originally Posted by Hartley118 View Post
I understand that in practice in the 1950s MW transmissions could achieve a 10kHz audio bandwidth and LW 8kHz. It woud be good to see some corroboration of those figures.
In this thread: https://vintage-radio.net/forum/showthread.php?t=120815, post #21des some documentary information (from Wireless World) on BBC AM bandwidths in the 1940s, 50s and 60s, including the fact that the Brookmans Park transmitters were at 15 kHz by 1947.

Later in the same thread is some information on the Droitwich LF transmitters, whose audio bandwidth appears to have been limited by aerial Q. But ±10 kHz was possible at LF, as evidenced by the French Allouis transmitter:

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Originally Posted by Hartley118 View Post
Certainly the better receivers of the 1930s-40s could get close to exploiting this bandwidth. For example, BBC Research Department tests on a Murphy A40C ( http://downloads.bbc.co.uk/rd/pubs/reports/1945-11.pdf ) indicated a frequency response to 9kHz within 6dB.
Post-WWII, provision for wide bandwidth AM reception appears to have become scarce in UK domestic radio receivers. From the major setmakers, the Murphy A186 and A188C seem to have been amongst the last of the kind. Their bandwidth in “wide” mode is unknown to me, but probably around 9 or 10 kHz; both had 9 kHz AF notch filters.

At the carriage trade end was the Dynatron Merlin console receiver, whose post-WWII variants successively use the T69, T99 and T139 tuners, all of which had >10 kHz AF bandwidth in their widest IF bandwidth setting.

At the hi-fi end of the market, wideband AM tuners remained available into the 1970s, although becomingly increasingly scarce from the mid-1950s onwards. In this thread: https://www.vintage-radio.net/forum/...ad.php?t=70496, post #17 included a list of hi-fi AM tuners and their AF bandwidths, where known. In the valve era, bandwidths went out to 12 kHz for some models, such as those from Quad.

(That list had some gaps that I’ve since been able to infill. The Armstrong AM44 was 5/9/12 kHz. The Jason AM/FM2 was 4/8 kHz. And one may add the Avantic BM612 at 4/8 kHz (as best I can tell, the latter was a clone of the Jason AM/FM2.))

In that list were the Australian Wright models:

Wright LDT-3A: Wide only, 14 kHz with fixed 10 kHz notch.
Allen Wright 2: 3.5 and 12 kHz (at -6 dB) with fixed 9 kHz notch.

The story is that Allen Wright developed the first version when challenged to build a solid-state AM tuner that would better the Quad AM3, regarded as something of a benchmark in the early 1970s. Retrospectively the designer remarked: “Both sounded like GOOD FM , just in mono, from the superb quality non commercial government stations. Compressed commercial stations sounded just that - compressed!”


Both Australia and NZ were late to adopt FM broadcasting (1980s), so some local makers continued to offer solid-state wideband hi-fi AM tuners through the 1970s. In NZ, AWA was one such, and its AM-3 model was also rebadged and sold in the USA by McKay Dymek, who later moved on to its own AM-5 design, which was followed by a series of unusual HF receivers. AWA also provided wideband AM in one of its three-piece music centres of the 1970s, its “Series 9”, although whether that was much appreciated by the buyers thereof was questionable. Back in the 1970s I was at a party in Wellington (one of those after-the-pub on Friday night deals) at which an AWA Series 9 was being used as the music source. As I was looking at it I was approached by a YL who said that she worked for AWA. When I commented upon the wideband AM feature she said that many customers were confused by it and did not know what it was for. So there might have been a sharp division in expectations between the hi-fi and general markets. AWA later entered into some kind of arrangement with Hitachi which saw the end of its own designs. The Hitachi-origin models had the usual dreadful narrow-band AM sections that one associates with Japanese equipment of the era. There seemed to be a lack of understanding that something better was required for markets that did not have FM. (That said, one or two Japanese makers, notably Sansui, sometimes did produce excellent AM equipment, but that was the exception.)


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Old 5th Sep 2018, 5:30 am   #31
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The BBC were seriously considering using wideband AM rather than FM on VHF in the 40s. They got as far as carrying out some test transmissions.
As I understand it the BBC was involved in two sets of FM vs. AM comparative testing.

The first set, planned towards the end of WWII, took place around 1946, and involved transmissions at both 45 and 90 MHz. The outcome was that FM was deemed to be better than AM. It was also found that there was no reason to depart from the established American FM deviation of ±75 kHz, but that a pre-emphasis of 50 µs was preferable to the original American value of 100 µs (which was reduced to 75 µs in 1945 with the change to the 88 to 108 MHz band.)

The second set took place in the early 1950s. In the interim, there was evidently industry lobbying (particularly by Pye) for AM, with BREMA getting involved. The argument was that AM might be more economical (at the receiver end only; it was certainly much more costly at the transmitter end), with an accompanying argument that even if it were not as good, second-best was acceptable for the UK. (Even though that notion conflicted with the “export-or-die” them then current.) The Beveridge Committee evidently handed the matter over to the TAC, with the BBC doing the testing for the TAC. Eventually the TAC decision was made in favour of FM. Apparently it was a three-way “contest”, amongst FM, AM wide band with noise limiting, and AM narrow-band without noise limiting. FM could provide a given quality and coverage of service at much lower service-area-boundary field strengths than were required for AM, and so required fewer transmitters for a given result.

Before the FM vs. AM issue was revisited, there might have been the impression that future VHF broadcasting in the UK would be FM. Circa 1949, one or two setmakers showed FM receivers, the HMV 1250 being notable. And I think that English Electric offered a TV-FM receiver.

Much has been written about this sequence of events, although scattered over various sources. As a single representative item that covers most of the bases, one may pick this letter to Wireless World from F.H. Beaumont of the R.N. Fitton (Ambassador Radio) company:

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Fitton designed and built – to the BBC’s requirements – the comparator receiver used in the BBC comparative tests. Beaumont wrote an IET paper about the design of this receiver, which is recommended reading.

As far as I know, the only other country that considered the AM option for VHF broadcasting was Belgium. Perhaps that was because it had chosen AM sound for its two TV systems (later designated as C and F).

In the TV case the choice was less critical though, given that transmitter coverage was effectively determined by the vision channel. To match that coverage with AM sound was easily done without excessive transmitter powers.

Some early British FM receiving equipment, issued before the decision in favour of FM was made, also provision for receiving VHF AM. At least Lowther, Chapman and Sound Sales advertised such VHF FM/VHF AM receivers in 1953-54. And as far as I know the prototype Quad FM of 1952 – not put into production - had AM capability as well. Lowther also offered a VHF-AM only tuner, and single-channel Band I VHF-AM tuners for TV sound circa 1953.

Perhaps one or two UK TV receivers in the valve era took some advantage of the high quality AM sound transmissions. The Dynatron TV32 could be a candidate here. High quality TV sound was also accessible via the Jason JTV FM-TV sound tuner of 1958, and its improved successor the JTV2. This appeared at the time when BBC experimental two-transmitter stereo transmissions used TV sound for one of the stereo channels, and FM for the other. UK-built TV sound tuners reappeared in 1971, with a UHF-only, FM-only model from Lowther and a UHF and/or VHF AM/FM model from Motion Electronics.

And finally an odd fact – the first non-experimental two-language TV transmissions used AM, not FM. The French devised a system in the mid-1950s that applied time-multiplex to the AM sound channel of system E transmissions in Algeria to convey both French and Arabic sound tracks simultaneously. AF bandwidth was limited to about 9 kHz, just under half the line frequency, because of the way the system worked.


Cheers,
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Old 5th Sep 2018, 10:31 am   #32
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Default Re: Historic AM audio quality

A friend of mine had a very nice Ferguson radiogram in the '50s and if you adjusted the tuning towards the edge of the centre frequency the HF response sounded sounded FM like.

Peter
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Old 5th Sep 2018, 10:42 am   #33
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All that does is to increase distortion. True, it sound like more top but isn't. I've used some recordings from a collector who was in the habit of doing this to plug gaps in FM recordings, and the change of timbre across the edit was a dead giveaway.
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Old 6th Sep 2018, 9:15 am   #34
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Thumbing through Radio Times editions from the 30's and 40's using the BBC Genome facility is eyeopening. In particular the programme time allocated to dance band, classical music was considerable and I wonder who was listening?

The reason I ask is that I remember watching the TV series "Turn Back Time" and looking at museum artefacts of how people lived and where I often ask "where is the radio?". The TV series "Call the midwives" a historical drama, which, except for the Doctor's home, shows lots of people living cheek by jowl in very small spaces. My inlaws, when children in the 20's shared a floor in a house and listening to the radio was not one of their pleasures. For many, adult life started at 14.

Wireless World, meanwhile, extol radio sets with 20 watts of something and a chassis on rubber mountings to stop micrphony on shortwaves!

This suggests that the esoterics of sound reproduction was limited to a small cohort of "buffs," for want of another word, who, notwithstanding the huge technical development of radio, once satisfied left a huge hole in the market for inexpensive basic radio if only it was worth listening to.

Any thoughts?
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Old 6th Sep 2018, 9:49 am   #35
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Thumbing through Radio Times editions from the 30's and 40's using the BBC Genome facility is eyeopening. In particular the programme time allocated to dance band, classical music was considerable and I wonder who was listening?
I suspect most people were. I'd be inclined to switch the radio on a bit longer if that were still the case. The BBC was delightfully limited for many years by the amount of records they could play and this resulted in some excellent orchestral and live music right up until the time the pirates persuaded the authorities to change the rules and that lowest form of musical life, the 'DJ' was born.

Les.
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Old 6th Sep 2018, 10:11 am   #36
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Why 6dB vs. 3dB... 3dB half power, 6dB half volts.
 
Old 6th Sep 2018, 10:14 am   #37
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All that does is to increase distortion. True, it sound like more top but isn't. I've used some recordings from a collector who was in the habit of doing this to plug gaps in FM recordings, and the change of timbre across the edit was a dead giveaway.
I don't doubt that it increases distortion, but if used carefully and the resulting sound is heard as an improvement by the listener, then in my eyes, there's nothing 'wrong' with doing that. Of course, it depends on how much you tune off centre towards the sideband, if you tune off just a tad, one can argue that you get a nice compromise. I do it with BBCR4 on LW, it's a broad as a gate and you have lots of scope to tune for 'best sound', transforming the sound from distinctly wooly to being crisper with no noticeable downgrade in the sound quality otherwise. There's no laws about tuning for best enjoyment factor, but lots of theory.

I do a similar thing - albeit for a different reason - when listening to two FM stations that are at close frequencies and particularly when the one you want to listen to is weak. By judiciously tuning off centre you can avoid the breakthrough of the stronger, adjacent station. So, for a few reasons, you don't have to 'centre tune'.
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Old 6th Sep 2018, 12:02 pm   #38
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I learned as a small child in the 50's and 60's listening to my crystal radio (very wideband response but terrible selectivity) that AM radio could have good fidelity. I listened through a "crystal" (piezo) earpiece whose treble response was excellent.

So with that point of reference, I was disappointed with the treble response coming from various more sophisticated radios. So I did the same detuning trick as Steve when recording shows off air from my Rotel tuner, and I still have some of the tapes. The tuner wasnt true wide band but it approached it and a slight detune restored the muted treble usefully. But the degree of detune depended on the amount of HF content in the programme itself and so had to be set carefully. What worked for one programme resulted in distortion in another. Recording to cassette meant the treble needed to be healthily there at the record stage, to mask the prominent tape hiss.
I found this detuning worked on some tuners and not others. On more highly selective tuners, it seemed only to result in splattering distortion.
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Old 6th Sep 2018, 12:08 pm   #39
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Exactly Tim, you simply don't tune off centre if the sound is noticeably downgraded as a result. It's a suck it and see method of getting the best sound.
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Old 6th Sep 2018, 12:26 pm   #40
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Further to 6db v 3db. I only mentioned that because exam and interviews would require you to answer....3db for example. Also if various manufacturers are quoting the performance of their various products certain standards should be observed.
However, as I have been told by various members, on a separate subject, this is not always the case and certain liberties were taken to make their products seem better that their competitors, that is all I was getting at.
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