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Old 17th Feb 2017, 3:51 am   #41
Synchrodyne
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Default Re: Dolby FM - why?!

Here is the Wireless World 1974 July article:

WW 197407 p.237,238 Dolby FM.pdf

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Originally Posted by Synchrodyne View Post
The FMX system had better intentions, in that the use in receivers of the noise reduction applied to the difference signal was optional, and “normal” signal processing was still available, in theory at least unmolested by the noise reduction pre-processing. I am not sure what stopped FMX in its tracks. The last article on it that I have seen (although I can’t say that I have looked that hard) was in Audio magazine for 1987 October, pages 127 and 128. At the time, it was poised “to go”, as it were.
I did find a couple of later articles in Radio-Electronics (R-E) that provide some clues as to the fate of FMX. The first, in the 1988 June issue, page 74ff, described some of the early changes (including reduced compression) made to get FMX to work properly and asserted that it was then bug-free. The second, in the 1989 October issue, page 52ff,covered detailed field testing done by MIT Labs and Bose, who concluded that in general, FMX degraded reception. Phase distortion and its effect on the correct demodulation of the quadrature subcarrier sidebands was one issue. (That sounds a bit like the NTSC colour TV problem.) The FMX folks, unsurprisingly, disagreed with those findings, but I suspect that that report could well have been what killed it.

Not to get off topic by dwelling upon FMX, an interesting and pertinent point is that although the 1988 article made a passing reference to Dolby (and also dbx) in explaining the principle behind the FMX noise reduction process, nothing at all was said about Dolby FM in either. That suggests that by 1988, Dolby FM was well in the past and perhaps forgotten about. From a technical perspective, one might have expected an article on a new FM stereo noise reduction proposal to have been compared with what went before, and perhaps even some commentary as to why the new system might succeed whereas the earlier one failed. Or maybe that would have been expecting too much from a magazine culture (not just R-E) that tended to focus on the "latest-and-greatest".


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Old 17th Feb 2017, 2:12 pm   #42
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Default Re: Dolby FM - why?!

Thanks for the extra .pdf Synchrodyne.

As has already been said Dolby FM got nowhere. However I was idly wondering whether it was still possible now to recreate the set-up just to hear what things sounded like.

All it would take is a good quality VHF/FM microtransmitter coupled up to some Dolby IC circuits suitably extracted from long-dead Dolby equipped cassette recorders. A change to pre-emphasis/de-emphasis time constants, some suitably large RF attenuators to put the selected radio receiver into a noise influenced situation and Voila!

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Old 18th Feb 2017, 1:11 am   #43
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Default Re: Dolby FM - why?!

That looks to be fairly straightforward. It would be necessary to line-up Dolby levels at both the transmitting and receiving ends. As far as I know, for Dolby FM, “Dolby level” was at 50% modulation, namely ±37.5 kHz deviation. As I recall, some FM tuners and receivers with a 25 microsecond de-emphasis option had built-in oscillators (at 400 Hz?) to allow line-up with both outboard Dolby B decoders and tape decks with Dolby B. Even where Dolby B decoders were built into FM tuners/receivers, that calibration facility would have been desirable when Dolby B decoding was bypassed to allow direct recording of the encoded signal.

Some information on line-up was provided in the Wireless World series of articles on the construction of a Dolby B encoder/decoder which ran in the 1975 May, June and July issues, and which is available in composite form here: http://www.keith-snook.info/wireless...%20reducer.pdf. The section on Dolby FM calibration is in the 1975 July issue, pages 317 & 318.


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Old 18th Feb 2017, 3:52 am   #44
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Default Re: Dolby FM - why?!

I have two FM tuners with built-in 400Hz calibration oscillators. I hadn't thought of Dolby as a potential reason. The ex-BBC Revox I assumed was for setting up levels in general, but I thought it was marketing window-dressing in the domestic Sony.

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Old 18th Feb 2017, 3:25 pm   #45
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Default Re: Dolby FM - why?!

... Oh, not forgetting the stereo MUX pilot tone filters for the decoding Dolby side ...

Hmm, I wonder if the extra stuff (RDS, etc) that is now put above the stereo difference part of the multiplex would mess things up, given the de-emphasis is only 25 usec? That is, in the highly unlikely event that the Dolby B system were to be used in anger today by the broadcasters. Clearly the home-brew experiment would not affected.

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Old 24th Feb 2017, 2:28 am   #46
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Default Re: Dolby FM - why?!

When Dolby FM arrived in the 1970s, it was normal for US FM broadcasters to also be transmitting an SCA subcarrier. In the stereo case, the SCA band was – at the time - 53 to 75 kHz, with 67 kHz being the usual SCA subcarrier frequency. Up to 10% of the maximum deviation was allocated to the SCA service. Thus one assumes that Dolby FM was designed with possible subcarrier interferences in mind.

And by then, it was frequently the case that FM stereo tuners and receivers included a filter (sometimes referred to as a “birdie filter”) between the discriminator and the stereo decoder that was both low pass, with say a 55 kHz turnover frequency, and band-stop, centred on 67 kHz, to provide additional SCA attenuation. And post-decoder filtering, presumably placed ahead of the Dolby decoder, usually was low-pass overall with notches at 19 and 38 kHz. So I’d say that overall the Dolby decoding action was fairly robust.

In the 1980s the FCC extended the SCA band to 99 kHz. Typically this allowed a second SCA subcarrier at 92 kHz. But the above-mentioned “birdie filter” would have provided good rejection of this, and anyway, by then Dolby FM was mostly in the past. RDS was probably fairly harmless. As far as I know it used the third harmonic of the pilot tone, i.e. 57 kHz, phased in such a way as not to add to the overall amplitude of the 19 kHz tone alone. And it had very narrow band modulation.

It is unlikely that Dolby would have been used with FM mono broadcasts, or that Dolby decoders would have been fitted to FM mono receivers. So the latter would have been protected against subcarrier interference by their customary 75 microsecond de-emphasis circuits. For those few tuners that had a separate switched mono feed from ahead of the stereo decoder, then perhaps the addition of a low-pass/band-stop filter could have been desirable in addition to 25 microsecond de-emphasis ahead of Dolby decoding.

Quite how Dolby level was managed in the presence of subcarriers is unknown, give that when they were present, the maximum modulation level allowed for the main stereo signal was reduced somewhat. From a transmission viewpoint, it may have been preferable to set Dolby level at say 6 dB below full-scale, in which case, in main carrier deviation terms, it would have varied with the allowed maximum modulation. But at the receiving end, a fixed proportion of maximum deviation, i.e. ±37.5 kHz, would have been easiest. Otherwise post-stereo decode, pre-Dolby decoding signal gain would have had to have been switched according to the incoming signal, except that the receiver would not have an accurate way of knowing the maximum modulation level for the incoming signal. SCA subcarrier detection and counting would help somewhat, but would not have been infallible.

Dolby did quite a bit of advertising of Dolby FM in “Audio” magazine in the 1970s. This one:

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seems to be something of a stretch though, at least in respect of the suggested home check using interstation noise. Given that the interstation noise is likely well above Dolby level, meaning that there is no Dolby expansion action going on, then the audible difference is simply a reflection of the difference between 75 and 25 microsecond de-emphasis curves. Quite a few tuners and receivers of the time had an additional un de-emphasized discriminator output for use with possible future quadraphonic decoders; one could demonstrate even better apparent high frequency headroom by using this.


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Old 11th Mar 2017, 5:41 am   #47
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Default Re: Dolby FM - why?!

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Originally Posted by Synchrodyne View Post
Regarding the static de-emphasis used with Dolby FM, it is surprising that the 25 microsecond number chosen to best match the American 75 microsecond standard for non-Dolby broadcasts was also replicated for use in Europe, where the non-Dolby standard was 50 microseconds. On its face, it looks as if it could have been simply carried across without further consideration. But it seems unlikely that both Dolby and the IBA would have neglected investigation of this aspect. Evidently some work by the German IRD (Institut für Rundfunktechnik) supported the 25 microsecond number. (See Wireless World 1976 August page 36).
But I have since come across an alternative position, as found in Gordon King’s “The Audio Handbook” of 1975, in which 17 microseconds was suggested (by whom is unknown) when Dolby was used in nominally 50 microsecond systems. That looks to be simply a linear derivation from the 25 microsecond/75 microsecond pairing.

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Old 11th Mar 2017, 6:01 am   #48
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Default Re: Dolby FM - why?!

17us simply moves the pole up to 59kHz, IE at the top end of the USB of the difference signal. So if srorecasting isn't included, it is essentially no deemphasis at all.

The other way of viewing it is that Dolby B is a dynamically varying preempasis/deemphasis system in itself, so why would you want another one running at the same time? The margin up to the peak deviation could be better spent on the clever preemphasis rather than on the fixed

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Old 11th Mar 2017, 11:57 pm   #49
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Default Re: Dolby FM - why?!

Wouldn’t 17 microseconds give a turnover frequency of 9.5 kHz, as compared with 3.18 kHz for 50 microseconds? Even so, it would still be virtually non-existent pre-emphasis, maybe around 4 dB lift at 15 kHz.

As I understand it, the retention of static pre-emphasis with Dolby B was to provide some measure of “compatibility” with non-Dolby receivers that had fixed 50 or 75 microsecond de-emphasis. Had that requirement not obtained, then I imagine that static pre-emphasis would not have been used at all, as you suggest. As far as I know, Dolby B itself did not use any static pre-emphasis (although DBX, at least as used in MTS, did.)

Perhaps for the 50 microsecond case it would have been possible to play around with the Dolby compression curves such that some semblance of compatibility could be obtained with non-Dolby receivers, whilst still satisfying the original noise reduction and increased dynamic range objectives. But then that would have violated another presumed objective, which was that the Dolby encoding be exactly the same as in the Dolby B system developed for domestic tape recording (open reel and cassette), so that decoding could be done within a tape deck that was part of a hi-fi system, and that Dolby FM broadcasts could be recorded without prior decoding, the latter being done on replay.


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Old 12th Mar 2017, 4:27 am   #50
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Default Re: Dolby FM - why?!

duh!

Yes. I'd forgotten a factor of 2*Pi Sorry, I must have been half asleep

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