[Speex-dev] Question from XM Radio

Carr, Terrance Terry.Carr at xmradio.com
Mon Nov 28 14:22:16 PST 2005

Thanks Jim, that's understood.  When I say AMBE isn't working well, I
only mean from the audience acceptance point of view.  Technically it is
fine.  It is exactly doing the job we had expected.

It's the long standing wish that everyone wants... More for less.  We
are just seeking a bit of magic that just may not be there.  Ideally
finding a codec that can perform tollerably/intelligibly at 2-4Kbps.
And still perform quite well for complex speech at 10-12kbps.

Just keeping my eyes (ears) out for something that I have never
considered before.

I really appreaciate the time you've taken to reply, Thanks.

  Terry Carr
  Manager, Broadcast Applications
  1500 Eckington Place, NE (Flr-2, Ops Mgt)
  Washington, DC 20002
  Phone: (202) 380-4081 - Fax: (202) 380-4768


* -----Original Message-----
* From: Jim Crichton [mailto:jim.crichton at comcast.net] 
* Sent: Monday, November 28, 2005 4:57 PM
* To: Carr, Terrance
* Cc: speex-dev at xiph.org
* Subject: Re: [Speex-dev] Question from XM Radio
* > On Sun, 2005-11-27 at 17:11 -0500, Carr, Terrance wrote:
* >> Thanks for the reply.  We are currently using AMBE (4Kbps) for our
* >> Traffic/Weather Channels.  If you have ever had a chance 
* to hear the
* >> service, you will know that AMBE does not do us well.
* >>
* >> I understand that 2Kbps is low quality, but any poorer than AMBE?
* >
* > If AMBE is 4 kbps, then most likely yes.
* >
* >> If can get a decent quality for other low bandwidth talk channels,
* >> such as about 10-16Kbps and have it sound rather clean, 
* then I would
* >> be gaining a bit of growth in the areas I'm seeking.
* >
* > 10-16 kbps is different and Speex is probably more useful there.
* AMBE has a pretty long history for low-bandwidth satellite 
* speech (the 
* predecessor IMBE is used in the Inmarsat-M system, and AMBE 
* is used in the 
* Inmarsat Mini-M and M4 at 3.6kbps and Iridium at 2.4 kbps).  
* A few years ago 
* I did an evaluation of 4kbps speech codecs for a 
* low-earth-orbit satellite 
* operator (now defunct).  AMBE+ at 4kbps was intended to be a 
* "toll quality" 
* codec, with MOS scores similar to ACELP 8kbps.  It was 
* clearly better than 
* the couple of other 4kbps codecs which we tested, and MUCH 
* better than 
* LPC10.
* Now, AMBE builds in forward error correction, which gives the most 
* redundancy to the most important vectors in the encoded 
* speech.  This is 
* optimized for circuit mode systems, where all bits are 
* delivered, even if 
* some are in error.  This sort of FEC is useless in packet 
* systems where the 
* data is either delivered correct or not at all.  If XM is 
* running speech in 
* a packet mode, perhaps AMBE is not handing the loss of blocks 
* of data data 
* well.  This might not be the fault of the codec itself, but 
* an issue with 
* the way that it is integrated into the system.
* Speex is unually flexible in its choice of bit and sampling 
* rates, and you 
* certainly have the ability to produce higher quality speech 
* than you can get 
* from AMBE.  But, as Jean-Marc points out, you will need a 
* higher bitrate to 
* do this.
* - Jim 

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