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Hugh,

"Thanks for your additional, very helpful comments. My comments are below, in-line.

Robert

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Quote Robert:

Well, I assume that that is no longer an issue. It's hard to imagine that it would be in this day and age, given modern electronics.

Quote Hugh:

Peak level control will always be required, whether the broadcast chain is analogue or digital (or hybrid). As long as analogue broadcasting continues (as it will here for another few years in TV, and indefinitely in radio), there will remain the need to prevent overmodulation, and digital systems must snever be allowed to clip, obviously.

I'm not sure what you mean by overmodulation, as the audio that accompanies an analogue TV signal is FM modulated, and one needs only keep frequency modulation of the audio carrier below a certain value. (I don't know what the maximum allowable modulation frequency is for analogue TV, but for FM radio in the USA, it's about 17 kHz.) However, modulation, as I just described it, is independent of amplitude, and yet your original comment was that one had to control amplitude to prevent overmodulation. Perhaps I'm missing something here.

Quote Robert:

I'm not sure I follow here... Do you mean, "so that the home viewer no longer 'wants' to adjust the volume"?

Quote Hugh:

No, I mean so they don't feel the need to adjust the volume between programmes or channels... They shouldn't have to... adjust it significantly when they switch to a different channel to watch a different programme.

Quote Robert:

If there is some standard specified by the broadcaster that correlates sound pressure level to signal level, then there shouldn't be a problem.

Quote Hugh:

There isn't and can't be.

Quote Robert:

So, for example, something like, "normal speach by a source within a few feet of the camera should produce average levels of around -x dBFS", or something like that.

Quote Hugh:

That is done, as I said above. Normal speech would typically register between PPM4 and PPM6 on a BBC-style peak meter, which is roughly -18 to -10dBFS.

Hugh, I intended my first statement ("correlates sound pressure level to signal level...") to be a generalization of the example given in my second statement ("normal speach...")

Quote Robert:

+8 dBu" means that the voltage of the signal, v, is 8 db above 1 microvolt, i.e. +8 = 20 log (v/0.000001).

Quote Hugh:

No. 0dBu is a reference voltage of 0.775V (RMS).

Actually, it's different here in the States, where we use "dBv" to mean your "dBu", and "dBu" here means yet something else. But my point here was only that such a figure alone does not specify the magnitude of a digital representation of a signal, despite all of the suggestions to the contrary earlier in this thread.

Quote Robert:

In practice, the PPM meter is probably much "dumber", not doing any calculus at all

Quote Hugh:

I think you'd be surprised just how complicated the electronics of an analogue PPM is -- and there definitely is calculus in the signal processing.

That means that it's sampling and then using a microprocessor to do maths. By the way, this isn't necessary, as you can always use an "analogue computer" to function as a meter, as was done in the past. So, for example, rather than calculating how much time is required to charge a capacitor, you simply charge a capacitor and measure how long it took.

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Robert Eidschun

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