[Asterisk-video] RE: Video transcoding

[John Martin]

Hi Olle, Sergio,
  It depends on how you do you're transcoding. The decoding is simple(ish) enough, it's the encoding that takes the time (in the order of 10 times longer to encode than decode). And what really takes the time with encoding is doing the video pre-filtering and motion estimation. When you're transcoding you can get away with almost none of these complex processes (though H.264 complicates things). Because you're doing a decode followed by an encode you can use the decoded motion vectors as input to the encoder - they may get scaled down or manipulated but you don't have to do O(N2) calculations to get them.
  Certainly a dedicated DSP (like the PixelWorks BSP-15 or TI 6446) would do a nice job of transcoding the video on a PCI card. Perhaps the SIMD pixel engines in a graphics card would be able to do something, but getting the manufacturers to support your code development might be tricky. Even finding register and instruction sets has been tough in the past. The MPEG chips on the graphics cards can be too MPEG and too decode only to do H.26x, but that's changing with the likes of TI and PixelWorks getting to lower cost points.

  And to move onto Sergios question on ffmpeg and the like. Those codecs are generally centred around a full decode and encode path. If you wanted a low-cpu usage transcoder then you'd have to break up the codecs and intercept some of the internal data paths. That's possible for sure, but may not get much support from the people maintaining the projects to keep those compilation options bug free... dunno, just my thoughts.

  As to whether it's a good idea to do this sort of thing on an Asterisk platform: Trancoders, and their big brother MCUs, cost upwards or $1000 a port (with perhaps 8, 12 or 16 port minimums), there's definitely a market for small low cost units for SME. You'd want to do the transcoding/MCU on a different box so as not to impact the main switch, but there's a demand I think.

John

John Martin
John.Martin at AuPix.com
http://www.AuPix.com


-----Original Message-----
From: asterisk-video-bounces at lists.digium.com [mailto:asterisk-video-bounces at lists.digium.com] On Behalf Of Olle E Johansson
Sent: 17 May 2006 08:28
To: Discussion of video media support in Asterisk
Subject: Re: [Asterisk-video] RE: Video transcoding


17 maj 2006 kl. 09.25 skrev Sergio García Murillo:


> And regarding video transcoding, it would be great to have some  
> capabilities integrated within asterisk so we could use the error  
> correction mechanism defined in AL3 to provide a much better video  
> quality.
> Does asterisk offer any capability? How it's handled in audio? Any  
> possibility of integrating libavocdec/ffmpeg or similar library?

My question is this:

- Is it realistic to do video transcoding on the platforms we run  
Asterisk on? How many channels?
- Would a graphics card, like the Hauppage with mpeg chips on it, help?

/O

_______________________________________________
--Bandwidth and Colocation provided by Easynews.com --

asterisk-video mailing list
To UNSUBSCRIBE or update options visit:
   http://lists.digium.com/mailman/listinfo/asterisk-video