Speex dev - Oct 2006 - 2 questions, frame size and SPEEX_GET_LOOKAHEAD

>
>> 2. What does SPEEX_GET_LOOKAHEAD do? How to use it?
>> In speexenc.c, there is following code:
>>
>> ...
>> speex_encoder_ctl(st, SPEEX_GET_LOOKAHEAD, &lookahead);
>> ...
>> nb_encoded = -lookahead;
>>
>> Can someone explain what this means?
>
> The lookahead is the number of samples you need to discard at the  
> start.
>
> 	Jean-Marc
In my application, even 5ms (110 samples at 22KHz) matters. So what  
should I do to avoid discarding samples at the beginning?
1. Turning off look ahead?
2. Padding 0's at the beginning.

> In my application, even 5ms (110 samples at 22KHz) matters. 
1) If 5 ms matters, I don't recommend Ogg (and I definitely hope you're
not running Windows!)
2) 22 kHz is *not* recommended. Use 16 kHz instead
> So what
> should I do to avoid discarding samples at the beginning?
Why are they so precious, they're *zeros* (or nearly).
> 1. Turning off look ahead?
> 2. Padding 0's at the beginning.
Or you can always just play them if they're so precious. Ah, the sound
of 5ms worth of zeros...

	Jean-Marc

Ok, let me first explain why 5ms matters, even they are 0's, in my  
particular application.

I am working on a speech synthesis system. The basic idea is  
concatenating pre-recorded phonemes or words into longer sentences. So  
any missing or extra samples, even it is as short as 5~10ms, cause  
very noticeable discontinuities.

I want to use speex to compress/decompress those pre-recorded  
material. But I'm concerned about the extra 0's might be padded at  
both ends.

For the zero padding at the last frame, I know how to remove it after  
decoding. But I am a little confused by the look ahead at the  
beginning. The sample code in the manual doesn't use look ahead, while  
the speexenc.c does. I'd like to know what difference it makes.

Let me plugin some numbers. I am using wide-band mode, the frame size  
is 320 samples. Say I take the first frame of an audio buffer, i.e.  
the first 320 samples, and feed them into encoder. Then after  
decompress, do I get all 320 samples, or a portion of 320 samples with  
some 0 padding at the very beginning?

Thank you.




On Oct 31, 2006, at 12:38 PM, Jean-Marc Valin wrote:
>> In my application, even 5ms (110 samples at 22KHz) matters.
>
> 1) If 5 ms matters, I don't recommend Ogg (and I definitely hope  
> you're
> not running Windows!)
> 2) 22 kHz is *not* recommended. Use 16 kHz instead
>
>> So what
>> should I do to avoid discarding samples at the beginning?
>
> Why are they so precious, they're *zeros* (or nearly).
>
>> 1. Turning off look ahead?
>> 2. Padding 0's at the beginning.
>
> Or you can always just play them if they're so precious. Ah, the sound
> of 5ms worth of zeros...
>
> 	Jean-Marc