Black Hole
May contain traces of nut
As a matter of curiosity I have been contemplating the process of taking a stereophonic audio signal and down-converting it to monophonic.
The obvious strategy is to sum the stereo channels (L+R)/2, but it seems to me that if the signals on L and R happened to be anti-phase, the result would be silence when the actual result of presenting the L and R signals to the L and R ears would not be silence.
FM stereo radio uses a system where a mono signal is sent on the main carrier, and a difference signal is sent on a sub-carrier. This means a mono-only receiver (or a stereo receiver switched to mono) picks up a mono signal by ignoring the sub-carrier. If I call the main signal S and the difference signal D, then if S = (L+R)/2 and D = (L-R)/2, L = S+D and R = S-D. Something similar is used on records: the sum signal is the left-right wiggle of the needle, and the difference is the up-down wiggle (or something like that) so that a mono player can easily obtain a mono signal by only taking the left-right wiggle.
The consequence of this is that in FM radio broadcasting (and record production) the sound engineers have to pay attention to how the sum and difference signals are created from the left and right signals. It's no good transmitting a signal which a mono receiver would pick up as silence.
I don't know how this is done, or even if it is a real problem at all - maybe these circumstances never arise in practice. Anybody know? T'web is unhelpful on this point.
The obvious strategy is to sum the stereo channels (L+R)/2, but it seems to me that if the signals on L and R happened to be anti-phase, the result would be silence when the actual result of presenting the L and R signals to the L and R ears would not be silence.
FM stereo radio uses a system where a mono signal is sent on the main carrier, and a difference signal is sent on a sub-carrier. This means a mono-only receiver (or a stereo receiver switched to mono) picks up a mono signal by ignoring the sub-carrier. If I call the main signal S and the difference signal D, then if S = (L+R)/2 and D = (L-R)/2, L = S+D and R = S-D. Something similar is used on records: the sum signal is the left-right wiggle of the needle, and the difference is the up-down wiggle (or something like that) so that a mono player can easily obtain a mono signal by only taking the left-right wiggle.
The consequence of this is that in FM radio broadcasting (and record production) the sound engineers have to pay attention to how the sum and difference signals are created from the left and right signals. It's no good transmitting a signal which a mono receiver would pick up as silence.
I don't know how this is done, or even if it is a real problem at all - maybe these circumstances never arise in practice. Anybody know? T'web is unhelpful on this point.