d3adf1sh
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here's from another article:
Influence of FLAC Compression Level on Sound Quality
In our previous series on computer audio (TAS Issues 218–221), we reported a sonic degradation when WAV files were compressed into the FLAC format. These results were disputed by those who claimed this was impossible since once the files were converted back to WAV, the files were bit identical. We made the same measurements on the digital files ourselves and obtained the same results. Yet we stood by our subjective results then and continue to do so now. The problem is that measuring digital files themselves has no bearing on the ambient jitter coming from a variety of sources generated within a computer, and it does not reveal audible sonic differences. Only when the digital information is transformed to analog by the DAC do these sonic differences become manifest. So we decided to ask whether we could make meaningful measurements using the subjective and objective methodology we described in Part 1 of this new series. The degradation that occurs when playing FLAC files over a range of different compression levels as assessed with these two methods is shown in Figure 1.
A serious degree of sonic degradation is heard when WAV files are compressed using FLAC. It appears to follow a regular, negative hyperbolic pattern, whether quantified subjectively or by the more objective technique of height measurement. These results agree with our previous finding that FLAC compression of a WAV file (at level 5 in dBPA) degrades playback sound quality (SQ) from a typical PC server (see TAS, Issue 220, Feb. 2012). Under the circumstances of this experiment, we found that the height method was significantly more sensitive than our subjective method. We also show the actual percentage compression at each setting within dBPA. Counterintuitively, it can be seen that there is no quantitative proportionality whatsoever between the degree of compression and sound degradation. We have also examined the effect of the so-called “U” or uncompressed setting on sound quality found on recent versions of dBPA. This setting is not the same as the “0” setting, and it produces a file about 1% larger than the original WAV file (data not shown). The sound quality of this file is still not equal to the parent WAV, but the degradation is close to the detection limits of either of our two methods, something on the order of about 5 points on our subjective scale and within 2 inches on our height scale. As for why this degradation occurs when using FLAC files, we can only speculate that real time format decompression somehow creates an extra drain on computer CPU resources that occurs concurrently with the digital- to-analog conversion step, presumably mediated by an increase in system jitter. Given the apparent audibility of pico-second and even femto-second jitter, perhaps it should come as no surprise that the ear can detect the effect of lossless decompression on SQ. The magnitude of this degradation might well vary with the number of other background processes running simultaneously on a given computer. The solution to this problem is to convert compressed files to WAV, store to the hard drive, and use these files for critical listening. This is what we have done for all of our high resolution downloads1. On the basis of these results, we strongly encourage all commercial download sites to offer the customer the purchase option of a completely un-manipulated WAV file or, for purposes of metadata convenience with some playback programs, minimally compressed FLAC files using the U setting found in dBPA, despite commercial resistance to following this advice.
www.theabsolutesound.com
Influence of FLAC Compression Level on Sound Quality
In our previous series on computer audio (TAS Issues 218–221), we reported a sonic degradation when WAV files were compressed into the FLAC format. These results were disputed by those who claimed this was impossible since once the files were converted back to WAV, the files were bit identical. We made the same measurements on the digital files ourselves and obtained the same results. Yet we stood by our subjective results then and continue to do so now. The problem is that measuring digital files themselves has no bearing on the ambient jitter coming from a variety of sources generated within a computer, and it does not reveal audible sonic differences. Only when the digital information is transformed to analog by the DAC do these sonic differences become manifest. So we decided to ask whether we could make meaningful measurements using the subjective and objective methodology we described in Part 1 of this new series. The degradation that occurs when playing FLAC files over a range of different compression levels as assessed with these two methods is shown in Figure 1.
A serious degree of sonic degradation is heard when WAV files are compressed using FLAC. It appears to follow a regular, negative hyperbolic pattern, whether quantified subjectively or by the more objective technique of height measurement. These results agree with our previous finding that FLAC compression of a WAV file (at level 5 in dBPA) degrades playback sound quality (SQ) from a typical PC server (see TAS, Issue 220, Feb. 2012). Under the circumstances of this experiment, we found that the height method was significantly more sensitive than our subjective method. We also show the actual percentage compression at each setting within dBPA. Counterintuitively, it can be seen that there is no quantitative proportionality whatsoever between the degree of compression and sound degradation. We have also examined the effect of the so-called “U” or uncompressed setting on sound quality found on recent versions of dBPA. This setting is not the same as the “0” setting, and it produces a file about 1% larger than the original WAV file (data not shown). The sound quality of this file is still not equal to the parent WAV, but the degradation is close to the detection limits of either of our two methods, something on the order of about 5 points on our subjective scale and within 2 inches on our height scale. As for why this degradation occurs when using FLAC files, we can only speculate that real time format decompression somehow creates an extra drain on computer CPU resources that occurs concurrently with the digital- to-analog conversion step, presumably mediated by an increase in system jitter. Given the apparent audibility of pico-second and even femto-second jitter, perhaps it should come as no surprise that the ear can detect the effect of lossless decompression on SQ. The magnitude of this degradation might well vary with the number of other background processes running simultaneously on a given computer. The solution to this problem is to convert compressed files to WAV, store to the hard drive, and use these files for critical listening. This is what we have done for all of our high resolution downloads1. On the basis of these results, we strongly encourage all commercial download sites to offer the customer the purchase option of a completely un-manipulated WAV file or, for purposes of metadata convenience with some playback programs, minimally compressed FLAC files using the U setting found in dBPA, despite commercial resistance to following this advice.
New Methods for Quantifying Sonic Performance: Part Two
Influence of FLAC Compression Level on Sound Quality In our previous series on computer audio (TAS Issues 218–221), we reported a sonic degradation when
www.theabsolutesound.com
