On Sat, 19 Jul 2003, Markus Kuhn wrote:
> All modern digital broadcast transmission systems introduce significant
> delays due to compression and coding. It is therefore common practice
> today that the studio clocks run a few seconds (say T = 10 s) early, and
> then the signal is delayed by digital buffers between the studio and the
> various transmitter chains for T minus the respective transmission and
> coding delay. This way, you can achieve that both analog terrestrial and
> digital satellite transmissions have rather synchronous audio and video.
> Otherwise, your neigbor would already cheer in from of his analogue TV
> set, while you still hear on DRM the "live" report about the football
> player aproaching the goal.
But that's exactly what does happen, analog TV is ahead of digital, often
leading to asynchronous cheering coming from different parts of the house.
> There are a couple of problem though with delayed "live":
>   - One is with the BBC. They insist for nostalgic reasons to transmit
>     the Big Bang sound live, which cannot be run 10 seconds early in
>     sync with the studio clock.
>   - Another are live telephone conversations with untrained members of the
>     radio audience who run a loud receiver next to the phone. The delay
>     eliminates the risk of feedback whisle, but it now ads echo and
>     human confusion. The former can be tackled with DSP techniques, the
>     latter is more tricky.
But then there's often a deliberate delay introduced so the editor can
push the cut-off button on the first f....

>   - The third problem is that in the present generation of digital
>     radio receivers (DAB, DRM, WorldSpace, etc.), the authors of the
>     spec neglected to standardize the exact buffer delay in the receiver.
Intestingly, I have noticed Radio 5 live is synchronous or even slightly
ahead of analogue on Digital Terrestial.   I put it down to relatively
instantaneous compression/decompression of audio cf video streams.
(NICAM is near-instantaneous on 15-year old technology)


Reply via email to