Hybrid routing describes routers providing integrated, simultaneous switching of audio and video signals. Given the widespread adoption of embedded audio equipment, the vast majority of broadcast facilities have been designed and installed with minimal, if any, audio routing capacity.
Hybrid routers switch audio and video signals together, with nearly perfect time coherence — something most easily done in a single frame. Any contemporary router control system provides breakaway control of mono audio signals, stereo AES-3 signals, Dolby E and Dolby Digital signals. Using the hybrid approach, these control systems should become easier to configure and use.
Requirements to produce theater-quality surround sound have grown simultaneously with consumer adoption of HDTV programming. Providing this capability in video production facilities involves adding audio-specific mixing equipment, production switchers or even recording consoles. (See Figure 1.) It also requires monitoring equipment and listening areas to evaluate surround-sound quality, and the ability to switch and control mono audio channels, discrete AES-3 signals, AES-3 signals with non-PCM payload and possibly even MADI signals used for bulk audio transport. (See Figure 2.) This is a significant expenditure of time, capital and equipment room space if audio is not embedded.
Modular equipment for embedding and de-embedding significantly reduces audio signal cabling but does not eliminate separate AES. This additional equipment also adds cost and consumes rack space.
Exact timing is required
Audio processing requires exact phase alignment of digital audio samples at 20.8µsec boundaries for 48kHz audio. Put differently, one sample of offset between two nominal 1kHz tones is equivalent to 7.5 degrees of phase offset error. HD-SDI embedded signals carry up to 16 mono channels of audio. MADI can carry 64. A final surround-sound program mix of six channels feeds the encoder. Often, 24 channels of audio, or more, are mixed down to the final six. Field recording for stage events requires audio consoles with multiple MADI I/O streams. Compared with traditional television sound, audio channel count has grown nearly tenfold and must all be perfectly aligned.