TV sound receiver RF/IF bandwidth - UK Vintage Radio Repair and Restoration Discussion Forum

Panrock · 27th Jul 2022, 11:09 pm

I am seeking opinions about what is the typical bandwidth of a vintage Band I 405-line TV SOUND tuner.

This question applies to both 'straight' TRF and superhet types. I am interested in determining the region beyond which there will be no 'significant' response to interfering signals. My own estimate would be a channel width of 'a few hundred kilohertz'.

Your thoughts?

Many thanks. Steve

Hybrid tellies · 28th Jul 2022, 9:53 am

I seem to remember the bandwidth of the 405 AM sound channel being either 100 or 200Khz wide. The reasons for this wide bandwidth was to reduce the effects of local oscillator drift and reduce the impact of impulsive noise.

On the transmitting side this meant that they would have to make sure they filter out any video sidebands and on the receiving end extra gain on the sound IF would be required for the wider bandwidth requirements.

Graham G3ZVT · 28th Jul 2022, 12:27 pm

Looking at this through the other end of the telescope,
What did the transmitted audio bandwidth curve look like compared with Long & Medium Wave practice (prior to modern "brick-wall" filters)?

marceljack · 28th Jul 2022, 6:28 pm

Quote:

Originally Posted by rambo1152

Looking at this through the other end of the telescope,
What did the transmitted audio bandwidth curve look like compared with Long & Medium Wave practice (prior to modern "brick-wall" filters)?

I see no reason to limit the audio bandwith on transmission side to less than 15 kHz (we had the same AM sound in France for both VHF 819 lines and UHF 625 lines transmissions).

G6Tanuki · 28th Jul 2022, 7:47 pm

Given that the first generation of TV receivers used a TRF configuration, your sound-channel receiver tuned to somewhere between 40 and 50MHz would inevitably have had a rather wide bandwidth - but that had potentially an advantage as far as audio quality was concerned.

Narrow-bandwidth amplifiers tend to have issues when faced with wide-bandwidth high-amplitude pulses [such as ignition interference from unsuppressed cars, or commutator-hash from motors] - they 'smear' the pulse so it becomes much more intrusively noticeable.

A wide-bandwidth amplifier chain causes less of this 'smearing' and so simple amplitude-limiters/clippers can be much more effective; this effect was used to advantage in both the viodeo and audio chains of first-generation TVs.

Interestingly, in the 1950s when the prospect of "VHF" audio-broadcasting reared its head, there was a big debate as to whether this should use FM or AM; Pye - given their experience with audio on 405-line VHF TVs - were at one time advocates of "wide band AM"...!

Nuvistor · 28th Jul 2022, 9:58 pm

As post #2, between 100khz and 200khz, to allow for LO drift and also to allow noise interference spikes to be narrow and allow a series pulse limiter to be used in the audio feed.

Source. Patchett Television Servicing Vol 1, although it does have a misprint leaving out the K in KHz. Oops.

peter_scott · 28th Jul 2022, 10:59 pm

Hi Steve,

I guess you are probably aware of these from EMI.

Peter

Synchrodyne · 29th Jul 2022, 12:26 am

In a Wireless World (WW) 1952 September article (1) on TV sound noise limiters it noted that it was usual to design the sound channel to pass all modulation frequencies up to at least 100 kHz, and without intermodulation between the audio and the interference pulses, in order to facilitate good noise limiter performance. That implied an sound channel bandwidth of at least 200 kHz.

An interesting point made in that article was that there was a trade-off between noise limiter effectiveness and net audio bandwidth (i.e. after the limiter). It was suggested that for “high fidelity”, the noise limiter constants be set for a 10 kHz audio bandwidth, whilst for situations that combined a high interference level with a low incoming signal strength, 3 kHz might be more appropriate.

In that regard, the AM sound TV systems that used pre-emphasis (Belgian C and F, with 50µs) might have been better off than those that had no pre-emphasis (UK A and French E and L), in that the noise-limiter roll-off could be used in lieu, or partially in lieu of the de-emphasis roll-off, allowing a noise limiter turnover somewhat below 10 kHz.

(I suppose that a latter-day approach to a TV sound noise limiter might have been to employ the TDA1001 (or similar) FM noise blanker IC.)

In a 1952 Pye IEE paper (2) on superhet TV receiver design, the sound channel was specified to have a bandwidth (-3 dB) of 250 to 500 kHz.

Some information on UK TV sound transmitted bandwidths is available here: https://www.vintage-radio.net/forum/...3&postcount=31.

(1) WW 1952 September, p.339ff, “Noise Limiters for TV Sound – Use of Germanium Rectifiers”, R.T. Lovelock (GEC)

(2) IEE Paper #1294, 1952 May, “The Design of a Superheterodyne Receiver for Television”, D.H. Fisher, P.A. Segrave and A.J. Watts (all from Pye).

Cheers,

Panrock · 29th Jul 2022, 9:22 am

Thanks to all responders. I wasn't expecting many would comment. And in particular, it's always an honour to be on the receiving end of Synchrodyne's scholarship.

Quote:

Originally Posted by peter_scott

Hi Steve, I guess you are probably aware of these from EMI.
Peter

Yes Peter.

Low frequency sound-radio-style IFs used with VHF television sound. How much sound tuning drift do you get on your 901? I recall about 1 'flat' of the control when I had my 702.

Steve

marceljack · 29th Jul 2022, 11:15 am

Quote:

Originally Posted by Synchrodyne

In that regard, the AM sound TV systems that used pre-emphasis (Belgian C and F, with 50µs) might have been better off than those that had no pre-emphasis (UK A and French E and L)...

I remember that during a certain period (in the 70s I think) Radio Luxembourg LW used a preemphasis (I dont know the time constant), the effect was really awful on most receivers (none of course had a deemphasis on AM).

Graham G3ZVT · 29th Jul 2022, 11:26 am

Quote:

Originally Posted by marceljack

remember that during a certain period (in the 70s I think) Radio Luxembourg LW used a preemphasis (I dont know the time constant), the effect was really awful on most receivers (none of course had a deemphasis on AM).

It did have a "bright" sound.
Wasn't there also some reverb applied to the studio mic?

peter_scott · 29th Jul 2022, 12:41 pm

Quote:

Originally Posted by Panrock

Yes Peter.

Low frequency sound-radio-style IFs used with VHF television sound. How much sound tuning drift do you get on your 901? I recall about 1 'flat' of the control when I had my 702.

Steve

Hi Steve,

Yes, 1 flat sounds about right but stable once warmed up.

Peter

28th Jul 2022, 12:27 pm	#3
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,713	Re: TV sound receiver RF/IF bandwidth Looking at this through the other end of the telescope, What did the transmitted audio bandwidth curve look like compared with Long & Medium Wave practice (prior to modern "brick-wall" filters)? __________________ -- Graham. G3ZVT

28th Jul 2022, 7:47 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: TV sound receiver RF/IF bandwidth Given that the first generation of TV receivers used a TRF configuration, your sound-channel receiver tuned to somewhere between 40 and 50MHz would inevitably have had a rather wide bandwidth - but that had potentially an advantage as far as audio quality was concerned. Narrow-bandwidth amplifiers tend to have issues when faced with wide-bandwidth high-amplitude pulses [such as ignition interference from unsuppressed cars, or commutator-hash from motors] - they 'smear' the pulse so it becomes much more intrusively noticeable. A wide-bandwidth amplifier chain causes less of this 'smearing' and so simple amplitude-limiters/clippers can be much more effective; this effect was used to advantage in both the viodeo and audio chains of first-generation TVs. Interestingly, in the 1950s when the prospect of "VHF" audio-broadcasting reared its head, there was a big debate as to whether this should use FM or AM; Pye - given their experience with audio on 405-line VHF TVs - were at one time advocates of "wide band AM"...! __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

28th Jul 2022, 9:58 pm	#6
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: TV sound receiver RF/IF bandwidth As post #2, between 100khz and 200khz, to allow for LO drift and also to allow noise interference spikes to be narrow and allow a series pulse limiter to be used in the audio feed. Source. Patchett Television Servicing Vol 1, although it does have a misprint leaving out the K in KHz. Oops. __________________ Frank

28th Jul 2022, 10:59 pm	#7
peter_scott Dekatron Join Date: May 2003 Location: Edinburgh, UK. Posts: 3,274	Re: TV sound receiver RF/IF bandwidth Hi Steve, I guess you are probably aware of these from EMI. Peter Attached Thumbnails __________________ http://www.nostalgiatech.co.uk https://www.youtube.com/c/PeterScott/videos Last edited by peter_scott; 28th Jul 2022 at 11:16 pm.

29th Jul 2022, 12:26 am	#8
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: TV sound receiver RF/IF bandwidth In a Wireless World (WW) 1952 September article (1) on TV sound noise limiters it noted that it was usual to design the sound channel to pass all modulation frequencies up to at least 100 kHz, and without intermodulation between the audio and the interference pulses, in order to facilitate good noise limiter performance. That implied an sound channel bandwidth of at least 200 kHz. An interesting point made in that article was that there was a trade-off between noise limiter effectiveness and net audio bandwidth (i.e. after the limiter). It was suggested that for “high fidelity”, the noise limiter constants be set for a 10 kHz audio bandwidth, whilst for situations that combined a high interference level with a low incoming signal strength, 3 kHz might be more appropriate. In that regard, the AM sound TV systems that used pre-emphasis (Belgian C and F, with 50µs) might have been better off than those that had no pre-emphasis (UK A and French E and L), in that the noise-limiter roll-off could be used in lieu, or partially in lieu of the de-emphasis roll-off, allowing a noise limiter turnover somewhat below 10 kHz. (I suppose that a latter-day approach to a TV sound noise limiter might have been to employ the TDA1001 (or similar) FM noise blanker IC.) In a 1952 Pye IEE paper (2) on superhet TV receiver design, the sound channel was specified to have a bandwidth (-3 dB) of 250 to 500 kHz. Some information on UK TV sound transmitted bandwidths is available here: https://www.vintage-radio.net/forum/...3&postcount=31. (1) WW 1952 September, p.339ff, “Noise Limiters for TV Sound – Use of Germanium Rectifiers”, R.T. Lovelock (GEC) (2) IEE Paper #1294, 1952 May, “The Design of a Superheterodyne Receiver for Television”, D.H. Fisher, P.A. Segrave and A.J. Watts (all from Pye). Cheers,

27th Jul 2022, 11:09 pm	#1
Panrock Nonode Join Date: Feb 2005 Location: Worcestershire, UK. Posts: 2,533	TV sound receiver RF/IF bandwidth I am seeking opinions about what is the typical bandwidth of a vintage Band I 405-line TV SOUND tuner. This question applies to both 'straight' TRF and superhet types. I am interested in determining the region beyond which there will be no 'significant' response to interfering signals. My own estimate would be a channel width of 'a few hundred kilohertz'. Your thoughts? Many thanks. Steve __________________ https://www.radiocraft.co.uk

28th Jul 2022, 9:53 am	#2
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: TV sound receiver RF/IF bandwidth I seem to remember the bandwidth of the 405 AM sound channel being either 100 or 200Khz wide. The reasons for this wide bandwidth was to reduce the effects of local oscillator drift and reduce the impact of impulsive noise. On the transmitting side this meant that they would have to make sure they filter out any video sidebands and on the receiving end extra gain on the sound IF would be required for the wider bandwidth requirements. __________________ Simon BVWS member Last edited by Station X; 28th Jul 2022 at 12:56 pm. Reason: Typo.