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Now I See the Origin of the Confusion......

Posted by Todd Krieger (A) on February 25, 2007 at 00:45:25

In Reply to: Believe it or not, zero-stuffing followed by a digital filter works posted by Christine Tham on February 24, 2007 at 21:50:17:

This is the first time I've seen "fuzzy" information on a chip data sheet, that could be interpreted totally wrong. Looking at the age of the data sheet, I think it was more an oversight than a deception.

It deals with the paragraphs and Figure 4. Starting at "To the Rescue"....

Figure 4 shows how oversampling filters work in both the time and frequency domains. We start with a sampled signal at 44.1 kHz (Figure 4a), which has images in the frequency domain (Figure 4b). The next step in the process is to increase the sample rate of the digital signal by inserting zero-valued samples (Figure 4c), resulting in the spectrum shown in Figure 4d.

The root of this whole problem deals with the "analog traces" superimposed with the digitized signals in Figure 4. Projecting the impression that "zero stuffing" would have no impact on the signal, at that particular stage, if it were to be converted to analog. Technically Figure 4c does show the "spikes" with a "zero signal," as I stated earlier. But the "analog waveform" pictured in Figure 4c does *not* depict what the waveform would look like if it were to be converted to analog at that particular state. It's just a representation of the **input** analog signal, to provide reference.

But instead of providing reference, the analog traces in Figure 4 provided confusion.

The one picture that is factually incorrect is Figure 4d. It would *not* be identical to Figure 4b. (Along with the analog trace, this induces the misconception.) There should be "much taller" spikes present at the sample frequency and all harmonics. (44.1 kHz, 88.2 kHz, etc.) Because the signal has been converted to a pulse train whose fundamental frequency is the base sample rate. (If I were to re-write this white paper, I'd remove Figure 4d altogether.)

The low pass interpolation filter, mentioned afterwards, is where the digital filtering actually takes place. The "zero values" does not filter anything. It's just a pre-stage for the digital filter. The final reason for the misconception is this is never mentioned explicitly.

The sad part is this ambiguity has caused a big misconception. Zero-stuffing being what solely occurs with oversampling. Which has gone as far as being accepted fact across the audio engineering community. (I've even heard this misconception applied to digital radio applications at a place I used to work at.)

And it may have even compromised or corrupted some designs along the way.

But at the time this white paper came out, I don't know if anybody would have known enough about oversampling (aside from those within the chip manufacturers themselves) to have caught this.

And since this has evolved into accepted misconception, the only way to realize it's a misconception is to literally "field strip" the individual elements that make up the oversampling/digital filtering process.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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