Brick Wall Digital Filters and Phase Deviations - page 2
The scope shot above is the reverse of the first test. The right channel is in black with a 20 kHz sine wave and the left channel is in green with a 1kHz sine wave. Note that the zero crossover points are still lined up exactly. We should also note that in each of the above scope photos that the 20 kHz sine wave is at a slightly lower amplitude than the 1kHz sine wave. Once again from our traditional analog electronics background and loudspeaker crossover background we should expect the 20 kHz sine wave to be out of phase, but as we can see this appears not to be the case. In the great bulk of CD players and outboard DAC's, an active low pass filter is used to smooth out the waveforms coming out of the DAC chip. One of the great properties of some types of active filters is that they will not introduce phase shifts in the band of interest. The two most commonly used for this application are Bessel and GIC active low pass filters. The one used for this converter is a GIC filter. I should also note that the unit tested has a standard 8 times over sampling FIR digital filter. There is no up sampling in this unit.
The reality of this situation, due to the lack of phase shift problems from FIR filters especially with supplementary low pass active filters and over sampling of CD digital playback systems, is that the next time you read some self proclaimed audio expert (usually on internet forums) stating that up sampling reduces problematic phase shifts of standard 44.1kHz / 16bit PCM audio, or some manufacturer claiming to have the magic digital antidote for phase shift in digital audio, take it with a grain of salt and point them to this article.
In addition, over the years I have had numerous cd players and outboard DAC's on my bench for test. The have been only two players that failed this test. One that was "modified" from the original unit, and the modification was done in the low pass filter section after the DAC. The " modification" caused the unit to be 7.5 db down at 20 kHz and a pronounced phase shift at 20 kHz. The second unit was a very old unit dating from the early 1980's that had no over sampling and an analog brick wall filter used for the low pass filter. This type of design was discontinued close to 20 years ago, and for good reason.
Check out this article for more information on over sampling .
The following four scope photos are expanded versions of the two used above.
Black Trace: 1kHz Sine Wave
Green Trace : 20kHz Sine Wave
Notice the perfect Sine Wave reproductions with no phase anomalies or distortion. At 20kHz we see a decrease in amplitude of about 100mVpp which represents about 0.1dB loss and is very common for a CD player with a FIR filter located slightly above ½ the Nyquist frequency (44kHz/2 = 22kHz).
Black Trace: 20kHz Sine Wave
Green Trace : 1kHz Sine Wave
Notice between every low and high peak of the 1kHz signal, there are 10
low and high peaks of the 20kHz signal. If we were to expand this out
to one full period, there would be 20 samples of the 20kHz for every 1
sample of the 1kHz signal.
Black Trace: 1kHz Sine Wave
Green Trace : 20kHz Sine Wave
Mathematically this is an exact ratio between a 1kHz and 20kHz signal. If there were phasing problems due to the FIR filter, this would not be possible to achieve.
Black Trace: 20kHz Sine Wave
Green Trace : 1kHz Sine Wave
Approximately 5 years ago I submitted to the audio magazine Bound For Sound copies of the first two scope photos, due to the fact that the editor/publisher was talking about "digital phase" in a number of his articles. The editor/publisher told me over the phone that a number of so called "experts" that he talked to said just the opposite about the "digital phase" issue. My reply was when they send me the scope photos I'll believe them.
I'm still waiting.
Incidentally, the editor/publisher of Bound for Sound never published those scope photos or offered to let me write an article countering those claims. I am truly thankful that Audioholics has allowed me to write this article and publish it on their web site.
Special thanks to Dan Banquer of R.E. Designs
www.redesignsaudio.com
