[H-GEN] Re: converting from speex to ogg vorbis

[David Jericho]

Geoff Shang wrote:

> Actually, the bass, also known as the LFE or subwoofer channel, is the 
> point 1.


It's a partial channel. It usually isn't encoded with enough detail to 
play sounds above the frequency range that an low frequency speaker 
usually deals with, being up to 120Hz. The second key difference with 
the .1 channel is that it's more of an information channel than an 
actual audio stream. It's often left to the decoder or amplifer as what 
to do with it as a sub may have a cut off below the 120 Hz mark. 
Sometimes sounds from the LFE channel will be played through the main 
speakers, low frequency noise often being difficult to localise.

As a side note, if your gear is THX compliant (and it is becoming more 
common), the LFE channel can be potentially played through through your 
main speakers. THX defines the sub woofer cut over point at 80Hz, while 
an LFE channel can range upto 120Hz. Other point of interest is that 
your surround speakers are dipolar, meaning the speakers are out of 
phase. Out of phase speakers have an "ethereal" quality to them, once 
again making it hard to localise the source of the sound. There are also 
two THX standards for home theatre, the key difference being the size of 
room the equipment can adequately service.

> Dolby Digital audio is encoded using Dolby's AC-3 codec.  this is a 
> lossy codec, like MP3 or Ogg Vorbis.  Its distinguishing feature, at 
> least from MP3, is that it is multi-channel.  
> http://www.dolby.com/tech/multipc.html


A very important difference. While MP3 encoders typically default to 
joint-stereo compression (i.e. the channels are masked against each 
other), AC3 has seperate stream compression, the added benefit being 
that imaging and seperation of sound out your speakers is far better.

> Dolby Digital 5.1 mixes are generally encoded on DVD at 448 kbps or 
> 384 kbps.


DTS as a side note encodes in a lossy format at 754 kbit or 1.5mbit/s.

-- 
David Jericho
Senior Systems Administrator, Tucana Technologies