[rtcweb] Audio transcoding: CPU costs

[Harald Alvestrand <[e-mail]>]

To return to the question of "cost of transcoding":

- AMR is less complex to en/decode than OPUS. So the dominant cost of 
transcoding between OPUS and AMR is the OPUS operation.
- http://www.ietf.org/mail-archive/web/rtcweb/current/msg05236.html 
estimates that OPUS en/decode can be done at ~1000 channels per CPU.
- One machine for one minute on Amazon AWS costs 0.24 cents for a 
medium-sized "high-CPU instance" according to 
http://aws.amazon.com/ec2/pricing/ (0.145 dollars per hour).
- Accordingly, transcoding, if done on fully loaded Amazon AWS 
instances, adds ~0.00024 cents per minute to the cost of a call.

There are other ways to provide such a service. It seems unlikely to 
increase the CPU cost of the call by more than a factor of 100, so we're 
still below 0.024 cents per hour.

I know there are other arguments for avoiding transcoding. But calling 
the CPU cost "negligible" seems appropriate in a world where mobile 
non-"all you can eat" plans still sell for tens of cents per minute.