Forums

Regarding comparison of audio quality

Started by vasindagi June 26, 2008
On Jun 26, 3:34&#2013266080;am, "vasindagi" <vish...@gmail.com> wrote:
> Hi, > I m working on QMF filters wherein I split the input audio into different > bands and then reconstruct them. I wanted to know how can I compare the > quality of the input audio and the reconstructed audio. > Right now I m using mean square error which I feel is not sensible because > the more the filter length I use more is the error that is showing up. > Can I use cross correlation to compare the qualities. Please suggest any > other quality measure. > I also wanted to know if I can compare the quality using the frequency > domain. > Thanks in advance.
V, Are all frequency bands subject to the same amount of delay? For instance if you make more QMF filter pair splits at low frequencies than at high frequencies, then they generally are not. If all of the filter lengths for a level of decimation are not the same length, then they generally are not. If you use QMF's for an analysis-synthesis system (with analysis and synthesis filters paired properly), you can put an integer delay (no fractional part required) in each output band (compensates for the combined analysis delay and synthesis delay). Delay can be placed on either the analysis or synthesis side (your choice). The big question is, what are you doing to the signals at each final frequency band output? That may determine whether a MSE (for SNR) will correlate to quality. Test your QMF implementation by including all bands, no processing (low-bit-rate quantization) of the final band outputs, and put an impulse into it. For reasonable QMF filter choices, the synthesized output should look pretty much like an impulse. If not, you have a problem. You should be able to predict the delay (integer samples, no fractional part) between the input impulse and reconstructed impulse; your results should match your prediction. Finally, remember that the QMF synthesized outputs will have aliasing from the matching QMF analysis (with decimation) cancelled, but there will be aliasing within each analysis frequency band. Did you notice that filter outputs may be flipped in frequency and not in the frequency band order one might initialy hope for? Dirk