Hi all,
In vocoders, why linear prediction filter is always designed as 10th order?
From where does the magic number 10 came from?
This question is cross posted from dsperado@dspe...
http://groups.google.com/group/dsperado/browse_frm/thread/6c2850f0ba4bd684/721849ed96f43992#721849ed96f43992
Satheesh Ram
LPC filter order
Started by ●January 19, 2006
Reply by ●January 19, 20062006-01-19
Satheesh..
> [Satheesh Ram <satheesh.ram@sath...]
> Subject: LPC filter order
>
> Hi all,
> In vocoders, why linear prediction filter is always
> designed as 10th order?
> From where does the magic number 10 came from?
Well, it's a 'rule of thumb':
number_of_LPC_terms = 4*bandwidth_in_KHz + 2
Why? Most (or at least many) voice codecs work with audio sampled at 8KHz (meaning 4kHz
bandwidth, but for practical telephony use, this is often restricted to 300-3.3kHz passband).
Now, for voiced speech, there's usually one formant peak per kilohertz of bandwidth.
It turns out you need two predictor terms to describe each formant peak - so there's 8 terms right
there. (I believe the original GSM vocoder used 8 LP terms.) Using an extra two extra LPC
coefficients further minimizes residual energy...
Bill Wiese
San Jose, CA USA
> [Satheesh Ram <satheesh.ram@sath...]
> Subject: LPC filter order
>
> Hi all,
> In vocoders, why linear prediction filter is always
> designed as 10th order?
> From where does the magic number 10 came from?
Well, it's a 'rule of thumb':
number_of_LPC_terms = 4*bandwidth_in_KHz + 2
Why? Most (or at least many) voice codecs work with audio sampled at 8KHz (meaning 4kHz
bandwidth, but for practical telephony use, this is often restricted to 300-3.3kHz passband).
Now, for voiced speech, there's usually one formant peak per kilohertz of bandwidth.
It turns out you need two predictor terms to describe each formant peak - so there's 8 terms right
there. (I believe the original GSM vocoder used 8 LP terms.) Using an extra two extra LPC
coefficients further minimizes residual energy...
Bill Wiese
San Jose, CA USA
