Electronic – Vestigial Side Band Demodulation: Which components are to be retained and which are to be filtered out

Your perfectly single-sideband suppressed-carrier modulated sinusoid certainly has a phase which can be measured. However, what you cannot tell is what the contributions of that measured phase from the audio input and the RF oscillator were.

There is another form of single-sideband modulation, in which not only one sideband but also the carrier component is transmitted. This provides a reference which can be used to synchronize the receive LO to the transmit one - normally done to insure exact tuning, but it would also give you the ability to recover the original audio phase.

It is also quite possible, especially with modern DSP gear, to transmit two separate audio channels, one on each side band. This is commonly called independent sideband modulation (ISB).

Many spread spectrum implementations are DSP based and capable of receiving multiple channels at once - GPS being a good example.

There's a trick that is used in NTSC that I believe also applies to PAL.

If you look at the fine detail of the spectrum of the luminance signal, you find that most of the energy is concentrated at multiples of the horizontal sweep frequency, with relatively little energy in between these peaks (this energy represents diagonal edges in the image, which are relatively rare). There is a similar pattern in the details of the chrominance signal. Therefore, the color subcarrier frequency was carefully chosen to place the peaks of its power spectrum between the peaks of the luminance spectrum.

The filters that can separate this information into two separate streams again are called "complementary comb filters". You create a comb filter for the luminance signal by delaying it by one horizontal line period and adding it to the original signal. You then subtract this filtered signal from the original signal to get the complement signal, which contains primarily the chrominance signal (which you subsequently bandpass filter to the appropriate range of frequencies).