15-08-2020, 11:13 AM
I'm not sure there is any sort of "official" convention here - so much depends on the context. I'd say that RMS is by far and away the most common - so therefore is the the safest assumption in the absence of any other guidance. But I see so many people making mistakes here that I always advise people to be pedantic about it - so specify whether it's RMS or peak or peak to peak unless it's really clear from the context.
If the context is related to power in any way, then the voltage must be RMS, as calculating watts based on peak or peak to peak will give wrong results. That means that any conversions involving power - such as from dBm to voltage - must be based on RMS quantities.
Most of the time in audio the measurement of interest is RMS, but converting to pk-to-pk and vice-versa is necessary for working out how much signal level you get for a given set of power supply rails, for example. Some program meters measure RMS or average, but others measure peak (most often quasi-peak) because overloading transmitters and other media is something to avoid - but peak levels tell you nothing about how loud an audio signal is, which obviously causes a quite specific sets of challenges in broadcasting.
However, if you're talking about analogue video signals, the RMS is not really meaningful. Peak to peak is the most interesting thing, or even just peak if you're ignoring the sync pulses. It's been a while since I've got my hands dirty with analogue video, so can't remember how APL (average picture level) is calculated, but I think it's a simple average of the values of the active picture area rather than an RMS calculation. I can't see the point of doing RMS for this - RMS is about equivalent DC heating effect*, which is not related to picture content in any way I'm aware of. To keep this thread on track, then a simple correction is welcome if I'm wrong about that, but if anyone wants to expand on it (more than a sentence), then please start a new thread.
* Many here will think I'm stating the utterly obvious - perhaps even being borderline patronising - but I have to cover this with all new recruits, and it's surprising how few know that RMS is the equivalent DC heating effect. <long rant about academia self-edited out!>
Another thing that again might be obvious or worse to some of the folk on here, but my teaching experience tells me it's worth the risk of saying it: the "magic" root-2 that gets you between RMS and peak is only valid for sine waves. Obvious when you understand the "equivalent DC heating effect" point, but well, see my previous paragraph
If the context is related to power in any way, then the voltage must be RMS, as calculating watts based on peak or peak to peak will give wrong results. That means that any conversions involving power - such as from dBm to voltage - must be based on RMS quantities.
Most of the time in audio the measurement of interest is RMS, but converting to pk-to-pk and vice-versa is necessary for working out how much signal level you get for a given set of power supply rails, for example. Some program meters measure RMS or average, but others measure peak (most often quasi-peak) because overloading transmitters and other media is something to avoid - but peak levels tell you nothing about how loud an audio signal is, which obviously causes a quite specific sets of challenges in broadcasting.
However, if you're talking about analogue video signals, the RMS is not really meaningful. Peak to peak is the most interesting thing, or even just peak if you're ignoring the sync pulses. It's been a while since I've got my hands dirty with analogue video, so can't remember how APL (average picture level) is calculated, but I think it's a simple average of the values of the active picture area rather than an RMS calculation. I can't see the point of doing RMS for this - RMS is about equivalent DC heating effect*, which is not related to picture content in any way I'm aware of. To keep this thread on track, then a simple correction is welcome if I'm wrong about that, but if anyone wants to expand on it (more than a sentence), then please start a new thread.
* Many here will think I'm stating the utterly obvious - perhaps even being borderline patronising - but I have to cover this with all new recruits, and it's surprising how few know that RMS is the equivalent DC heating effect. <long rant about academia self-edited out!>
Another thing that again might be obvious or worse to some of the folk on here, but my teaching experience tells me it's worth the risk of saying it: the "magic" root-2 that gets you between RMS and peak is only valid for sine waves. Obvious when you understand the "equivalent DC heating effect" point, but well, see my previous paragraph
