Re: [codec] #16: Multicast?

"Raymond (Juin-Hwey) Chen" <> Mon, 26 April 2010 21:02 UTC

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From: "Raymond (Juin-Hwey) Chen" <>
To: Mikael Abrahamsson <>
Date: Mon, 26 Apr 2010 14:02:18 -0700
Thread-Topic: [codec] #16: Multicast?
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Subject: Re: [codec] #16: Multicast?
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Hi Mikael,

Thanks for sharing your experience using a BT headset for Skype calls.  I think part of the quality degradation that you experienced was due to the reduction of the audio bandwidth to narrowband (8 kHz sampling, 3.4 kHz bandwidth), and another part of the degradation was due to the audible coding noise of the CVSD codec, a 40-year-old coding technology first proposed in 1970.  

If a high-quality, low-complexity, wider bandwidth IETF codec mode can be implemented in Skype and the Bluetooth headset to avoid the CVSD transcoding (together with wideband upgrade of the transducers and audio path in the BT headset, of course), then not only will you get much better speech quality in your Skype calls than what you have experienced, but also you will get a lower latency.  This is because transcoding between the Skype codec and CVSD not only accumulates the coding distortion of the two codecs, but also accumulates the coding delays.  Although CVSD is a sample-by-sample codec, BT headsets still transmit the CVSD bit-stream in 3.75 ms or 7.5 ms packets, and they can potentially add a one-way delay up to 20 ~ 25 ms through the Bluetooth headset (the exact delay depends on the implementation).

While we were discussing whether a 5 ms packet size can even be considered, for many years Bluetooth headsets have been using an even smaller 3.75 ms packet size.

Best Regards,


-----Original Message-----
From: Mikael Abrahamsson [] 
Sent: Sunday, April 25, 2010 1:06 AM
To: Raymond (Juin-Hwey) Chen
Cc: Koen Vos;
Subject: Re: [codec] #16: Multicast?

On Sat, 24 Apr 2010, Raymond (Juin-Hwey) Chen wrote:

> (7) Already a lot of people are used to using Bluetooth headsets to make 
> phone calls today.  If they have a choice, many of these people will 
> also want to use Bluetooth headsets to make Internet phone calls, not 
> only through computers, but also through smart phones connected to WiFi 
> or cellular networks.  As more and more states and countries pass laws 
> to ban the use of cell phones that are not in hands-free mode while 
> driving, the number of Bluetooth headset users will only increase with 
> time, and many of them will want to make Internet-based phone calls.

I purchased a BT headset with the anticipation of using it with my 
computer to make Skype calls. I tried it, but the sound quality when doing 
bidirectional audio (whatever that mode is called) is not good enough, it 
worsens the "skype IP" call quality. I agree that the use case is 
interesting, but as long as BT sound quality is what it is, it's really 
only the "low end" type  sound quality we're talking about.

But yes, I make skype IP calls with my Nokia N900 using BT sometimes, so 
the use case example is definitely valid.

Mikael Abrahamsson    email: