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Doppler encode/decode
Doppler encode/decode
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Old 3rd January 2002, 01:04 AM   #1
Bill F. is offline Bill F.  United States
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Lightbulb Doppler encode/decode

So I'm sitting on my inlaws' couch on Christmas vacation, and I'm wondering about doppler distortion. (All you who share this audio sickness will recognize the randomness, social inappropriateness, and incongruity of time and place of these musings—what can I say, that's where it happened)

I've been wondering how and to what degree doppler distortion really manifests in audio reproduction.

My limited understanding runs thusly:

The complete audible spectrum of sound waves vibrate the diaphragm of one or more microphones which are mixed down to a stereo pair of full-range signals that can be visualized as grooves on a record: the high frequencies ride the low ones and are frequency modulated by them. Bass signals are carrier waves for treble. I guess this inherent modulation is what some call doppler encoding.

Now, the fact that there is doppler information inherent in every recording implies that a certain exact amount of doppler shift is necessary during playback to properly decode the information and truthfully reproduce the original sound.

Planted deep in my inlaws' couch, staring straight ahead and oblivious to pleasant conversation, I distilled the simple truism below. Perhaps it's already been stated centuries ago, but if not, I'll humbly call it "Bill's law:"

For any given piece of recorded music, to achieve correct doppler decoding with a full-range speaker tranducer, the ratio of the speaker tranducer area to max amplitude must match that of the microphone(s) that recorded the music.

I'll call this ratio Dm for the microphone and Ds for the speaker

Since it was a comfortable couch, I also squeezed out a corollary.


During playback of a given piece of recorded music, proper doppler decoding will occur only at a certain loudness setting determined by the area of the speaker transducer (where Dm=Ds).

(I am unread in sound science, so somebody stop me if I'm taking credit for something straight outta loudspeaker 101, but I'm quite proud of my little law.)

So here's what I'm still wondering and would love some feedback on:

1. Relative to other forms of distortion, how audible/important is doppler distortion?

2. If doppler coherence IS important, what damage is done by a crossover that strips the bass carrier signal out of mid and treble, leaving the recording's doppler information undecoded? (Full-rangers rejoice!--up to a certain volume level)

3. Would it not be useful to know the Dm of the microphone that recorded your music, so you could match it with your speakers' Ds and begin to hunt for that elusive pinpoint volume level where net doppler distortion is exactly zero? (wouldn't that be great, wouldn't that be cool, huh?)

4. Where on earth will this thread lead? To the final triumph of full-range speakers? To bandwith-limited bass, mid, treble microphones feeding seperate tracks on a recording?


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Old 8th January 2002, 05:08 PM   #2
Bill F. is offline Bill F.  United States
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The silence is deafening. No one wants to fill me in on the whys and wherefors of doppler distortion?

I remain doggedly curious about how much it really shows up in audio reproduction.

Anyone willing to point me to a link or a book that discusses this most un-discussion-inspiring phenomenon?
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Old 8th January 2002, 10:12 PM   #3
Won is offline Won
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My guess is that doppler distortion (mentioned in TubeCAD, somewhere recently) is a possible source of intermodulation distortion. ID, in noticeable quantities, is quite objectionable because it is non-harmonic. However, because so-called doppler distortion is occuring on a microphone where the diaphragm displacement is exceedingly low, the amount of modulation that is occuring is probably quite minimal in comparison to other sources of ID.

Not being a recording engineer, I would have to look into the specifics, but my trustworthy spider-sense tells me that I'm not going to lose sleep over doppler distortion, at least at the recording end. In general, recording tech is higher fidelity than reproduction tech. Consider the scale difference between a microphone diaphragm's weight/displacement to a midrange driver's. There are some orders of magnitudes to be made up in the reproduction end.

Although, as an interesting mathematical exercise, I wonder how hard it would be to de-intermodulate a signal if you knew the displacements. Hrm. If it were, this is something that could happen in the studio, and we consumers shouldn't have to worry about it. "Bi-mic-ing" is an interesting thought, but I don't think we'll ever get to see those bandlimited tracks.

As to full-range drivers...I think that these are yet another approach to eliminating the passive crossover. However, I think multi-amping is a superior approach. Divide and conquer...

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Old 8th January 2002, 10:22 PM   #4
Won is offline Won
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This may be a relavent link, but I haven't time now to fully check it out...

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