Las Vegas – ShoreTel (www.shoretel.com) has announced that the latest release of its phone, er UC software now supports HD audio conferencing. The bigger question is how many HD voice codecs the software/hardware IP PBX system actually supports.
A conversation on the Interop show floor with a sales engineer lead to a setup screen that listed “high definition” codecs, with G.722, “DVI4,” “L16,” “BV32,” and “AAC” – the last taking a whopping 256 Kbps. Since ShoreTel phone hardware is engineered to work with “7 octaves of sound” – as anyone who works for the company will tell you after 2-4 questions – this would imply that one could actually make AAC phone calls if the system is configured for it.
“BV32” presumably refers to the open-source Broadvoice 32 wideband codec rolled out by Broadcom a couple of years ago. Further investigation through a ShoreTel online manual indicated “DVI4” referred to the ADPCM codec while searching for a definition of L16 only brought up “Linear.”
AAC is the most interesting codec listed. Designed as a successor to MP3, AAC samples in a range from 8 to 96 kHz and better handles audio frequencies above 16 kHz. It’s currently used in the Apple iFamily (iTunes, iPod, iPhone, iPad). .
L16 is simply 16 bit signed linear PCM, aka uncompressed PCM
http://en.wikipedia.org/wiki/RTP_audio_video_profile references RFC 3551 re RTP:
4.5.11 L16
L16 denotes uncompressed audio data samples, using 16-bit signed
representation with 65,535 equally divided steps between minimum and
maximum signal level, ranging from -32,768 to 32,767. The value is
represented in two’s complement notation and transmitted in network
byte order (most significant byte first).
The MIME registration for L16 in RFC 3555 [7] specifies parameters
that MAY be used with MIME or SDP to indicate that analog pre-
emphasis was applied to the signal before quantization or to indicate
that a multiple-channel audio stream follows a different channel
ordering convention than is specified in Section 4.1.
…upon reflection, I guess that makes L16 “The Un-codec”
I can find nothing the stipulates any particular sample rates for L16. It seems that any commonly used sample rate is A-OK.