Re: QDM* IMDCT

Sascha Sommer wrote:

>>Hi,
>>I did a little digging in QDesign tonight and found an interesting
>>function. I'm no expert on the matter, but I think this may be one of them
>>IMDCT doohickeys that are so prevalent in perceptual audio codecs. Can
>>anyone "skilled in the art" (as they say in the patent literature) confirm
>>or deny, compare or contrast with known IMDCTs? Note that I may have a few
>>functional details mixed up, so don't take it as authoritative (and a few
>>detals are missing at the end right now).
>>
>>    
>>
>
>You probably already compared this with dct36.c in mp3lib but are the
>constants cosinus values?
>
After long wasted time, how the h*** can you convert single presicion 
float in dword hexform to a base 10 float that I can understand.

This sort of works but I want to be able to test many different 
hexstrings without recompiling, and the other way around would be nice 
to know also:

void main(){
        unsigned int x = 0xABCDEF123;
        printf("%f", *(float *)&x);

}


Anyway I found a table with ca 125 values:

0FB17F3F = 0.998795      Beginning
30FB48BD = -0.049068
6DC47E3F = 0.995185
.
.
.
D86B67BF         midle
EA5AF1BE
98C561BF
3D9C03BF
.
.
.
F85378BF      end
CCCF78BE
BE147BBF
C2C547BE
AC3A7DBF = -0.000000


Anyway if someone could help with some program that easily could convert 
hex to understandable numbers that would be most helpfull. I think this 
could be a cos table.

>
>
>Sascha
>  
>



The step after finding the structure I found earlier is that the decoder 
starts shifting out bits and doing stuff.


Anyway thanks Mike and Sascha for the good support. Think we found 
another technique used in qdm*. Something is atleast done in 64byte 
blocks in qdm*, I guess that 64bytes should suffice for a usefull 
frequency resolution. Maybe it even matches the reduction that the 
quantisation(ulaw) imposes. Anyway I don't think it will take more then 
a few more weeks before the codec is reverse engineered. I will revise 
my paper when I have the time. I have some more details that I will 
update the paper with.

/Benjamin



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