RE: [AVT] RE: <draft-ietf-avt-rtp-vmr-wb-03.txt>: sampling rate

Hi Qiaobing,

> Is it true that all the coded frames output from a VMR-WB 
> __encoder__ use the 12.8k sampling 
> rate, independent of the original sampling rate of the speech?

The above statement is true. However, I want to make sure that it is not misinterpreted.

The VMR-WB encoder converts the 8 or 16 kHz sampled input speech to 12.8 kHz prior to the encoding functions. This INTERNAL sampling frequency is transparent (hidden) to the user. The bit stream generated by the encoder is then transmitted to the VMR-WB decoder.

The VMR-WB decoding functions are independent of the encoder input speech sampling frequency. By default, the VMR-WB decoder generates a wideband output, unless instructed otherwise. The internal sampling frequency must now be converted to 16 kHz (for wideband output) and the higher frequency band (6.4 to 7 kHz spectrum) must be reconstructed by the decoder. If a narrowband output is desired then 12.8 kHz sampling frequency must be converted to 8 kHz. Therefore, you CANNOT use the 12.8 kHz internal sampling frequency for any other purposes than the encoding-decoding functions. 

Depending on the output audio interface (or the network interface), one may wish to instruct the decoder to generate a narrowband or wideband output.

For proper operation, the RTP timestamp clock rate must be either 8000 or 16000 depending on the narrowband or wideband operation, respectively. The 12800 Hz internal sampling rate CANNOT be used for the RTP timestamp clock rate. The correct timestamp or clock rate (8000 or 16000) is required for proper buffering and other functions in the transmitting and receiving sites.

cdma2000 Service Option 62 (VMR-WB) also recognizes only 8000 or 16000 Hz sampling frequencies.

Since VMR-WB and AMR-WB codecs share the same core technology, the concept of 12800 Hz internal sampling frequency is used in both codecs. As you see in AMR-WB RFC and 3GPP specs, there is no external usage of the internal sampling frequency and the default RTP clock rate for the AMR-WB codec is 16000 Hz.

Regards

-Sassan Ahmadi

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