Audio Subcarrier on 1.2 GHz Transmitter

No, I think it is above your video carrier by 4.5 MHz. like the chroma burst which is also above the carrier by 3.58 MHz. The whole channel should occupy a band of about 6 MHz. But I might be wrong, this is from when I worked for Zenith in the late '80s.

Regards

Your AV receiver has two detectors, video (AM) and audio (FM). Your SDR has both but only one can be used at a time. Without a video signal your transmitter only modulates the FM signal so the SDR has no problem receiving it.

When the video input is active also then your SDR's FM detector is probably confused by the additional sidebands provided by the AM video signal. Messing with the IF bandwidth as you have asked about previously may exacerbate the problem since the video portion occupies 4.5 MHz vs. only 30kHz for the audio of the total 6 MHz composite a/v signal.

The distance between transmitter and receiver will also have a significant effect on how well your SDR handles the potential front-end overloading, try separating them to see if your radio can detect the FM portion properly.

I tried the sdr in all modes and it could not find the audio when the video was there. When I remove the video signal, the audio can be picked up easily so this transmitter must mix the audio in with the video. Also it states in the specifications that the transmitter is an FM transmitter.

Another interesting experiment I tried was transmitting a stereo digital audio signal from the digital output on the computer. This worked with a maximum quality of 24 bit 48000 Hz with no problems at all. The higher settings just resulted in silence coming from the receiver. This means that these little video transmitters are also great for digital modes too.

Maybe this snippet from Wiipedia will help you understand how and where the video/audio signals are combined for broadcast:

Transmission modulation scheme Spectrum of a System M television channel with NTSC color.

An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz. The actual video signal, which is amplitude-modulated, is transmitted between 500 kHz and 5.45 MHz above the lower bound of the channel. The video carrier is 1.25 MHz above the lower bound of the channel. Like most AM signals, the video carrier generates two sidebands, one above the carrier and one below. The sidebands are each 4.2 MHz wide. The entire upper sideband is transmitted, but only 1.25 MHz of the lower sideband, known as a vestigial sideband, is transmitted. The color subcarrier, as noted above, is 3.579545 MHz above the video carrier, and is quadrature-amplitude-modulated with a suppressed carrier. The audio signal is frequency-modulated, like the audio signals broadcast by FM radio stations in the 88-108 MHz band, but with a ±25 kHz maximum frequency swing, as opposed to ±75 kHz as is used on the FM band. The main audio carrier is 4.5 MHz above the video carrier, making it 250 kHz below the top of the channel. Sometimes a channel may contain an MTS signal, which offers more than one audio signal by adding one or two subcarriers on the audio signal, each synchronized to a multiple of the line frequency. This is normally the case when stereo audio and/or second audio program signals are used. The same extensions are used in ATSC, where the ATSC digital carrier is broadcast at 1.31 MHz above the lower bound of the channel.

Strange. I can sind the audio above the video where you say it should be but still when I add the video, the audio is wiped out by video and just becomes buzzing but yet the video receiver still works with it.

Not strange, that's because the video receiver is designed to demodulate the audio and video streams simultaneously while the SDR is not and gets "confused" if it tries. Consult the ARRL Handbook for more info.