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Topic: PQF (Read 2825 times) previous topic - next topic
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PQF

Im currently trying to understand the encoding process in MP3.

Ive read all I can and am a little unclear on a couple of things.

1. The 32 band PQF, ive just read that its not a series of band pass filters that are fed the PCM. Is this true and if so how do the filters work?

2. Also ive read that the bands overlap by 50% in the PQF. All well and good but surely this means that with band width of each filter being approx. 689Hz (22050 / 32), the overlapping will cause the bank to only reach 11,025Hz.

3. When a MDCT converts the time domain samples into frequency domain, does this literally mean that the sample rather than having a value for frequency in reference to time has a value for time in reference to frequency?

Im sure there'll be more soon!

Thanks

PQF

Reply #1 –
1. I don't really know the internals of the filterbank...

2. The filters do overlap, however the "width" that you give is only where each filter has maximum influence.  In reality, each filter will pick up parts of the other frequencies. So basically, the frequency domain is split up into 32 bands, and each filter picks up its band and a bit from the two on either side of it. I'd post a picture of this, but I can't seem to find a good one...

3. Not really. The "time domain" has a value for amplitude in reference to time. The frequency domain is similar: it's got values for amplitude in reference to frequency.
"We demand rigidly defined areas of doubt and uncertainty!" - Vroomfondel, H2G2

PQF

Reply #2 –
Quote from: MattStan on
Im currently trying to understand the encoding process in MP3.

Ive read all I can and am a little unclear on a couple of things.

1. The 32 band PQF, ive just read that its not a series of band pass filters that are fed the PCM. Is this true and if so how do the filters work?

2. Also ive read that the bands overlap by 50% in the PQF. All well and good but surely this means that with band width of each filter being approx. 689Hz (22050 / 32), the overlapping will cause the bank to only reach 11,025Hz.

3. When a MDCT converts the time domain samples into frequency domain, does this literally mean that the sample rather than having a value for frequency in reference to time has a value for time in reference to frequency?

Im sure there'll be more soon!

Thanks



Hi,
    The calculations in polyphase implimentation is another way to impliment the band of filters. I have got one pdf of David Pan. If you give me your rmail, I will send it to you.