Topic: Please help me understand/apply "stereophonic zoom"

[stevetoney]

I'm not sure I agree that one technique is more accurate than another in replication of sound placement.  Regardless, what helps me to understand the differences in X/Y coincident versus spaced patterns is that, in X/Y the stereo image is created by difference in sound pressure, while in spaced patterns the stereo image is created by timing differences...or delay caused by the fact that sound reaches one mic at a different time than the other.

In the SZ article, this is captured in secton 1.2 which says...

...localization of a sound source between the loudspeakers is obtained:

- by varying the intensity ratio between the two loudspeakers
- or by creating a time difference between them
- or by a combination of both intensity and time difference.

The best imaging technique might be different if you are listenign to stereo (two) speakers or headphones. In the excerpt from an article below Blumlein believed that the time differences in non-coincident microphone configurations would interfere with the time differences generated by the two playback speakers and reduce the accuracy of the image.

Excerpt taken from the article "Stereo Microphone Techniques Explained"
http://www.soundonsound.com/sos/1997_articles/feb97/stereomiking.html

Blumlein developed coincident techniques to overcome the inherent deficiencies (as he saw them) of the spaced microphone systems being developed in America. Since our hearing mechanism relies heavily on timing information, Dr Harvey Fletcher thought it reasonable to use microphones to capture similar timing differences, and that is exactly what the spaced microphone system does. However, when sound is replayed over loudspeakers, both ears hear both speakers, so we actually receive a very complex pattern of timing differences, involving the real timing differences from each speaker to both ears, plus the recorded timing differences from the microphones. This arrangement tends to produce rather vague positional information, and if the two channels are combined to produce a mono signal, comb-filtering effects can often be heard. Blumlein demonstrated that by using only the amplitude differences between the two loudspeakers, it was possible to fool the human hearing system into translating these into perceived timing differences, and hence stable and accurate image positions.

Definitely valid points Glenn and something to remember when listening to our recordings and reaching conclusions.  IOW, will you always reach the same conclusion about soundstage when you're evaluating a recording through headphones as you would if you're listening to the same recording through speakers that may not be perfectly balanced and which you aren't perfectly centered between.  Based on the quoted paragraph, the answer is probably 'no'.

[topdog]

My current understanding is that you will *never* get the same soundstage when listening to speakers verses listening via headphones for one simple reason; with speakers your ears hear both sound sources (left and right) but with a delay from the opposite channel which helps position things. But with headphones your ears only hear one source - left or right channel per ear. That creates a totally different soundstage interpretation by your ears/brain. So I think that the soundstage (from the same recording) as a minimum will always be wider (pulled to the sides away from center) when using headphones due to the lack of time delay which one would heard from the other channel if listening to speakers which would move images potentially more into the center of the soundstage. At least that's my current understanding.

[stevetoney]

My current understanding is that you will *never* get the same soundstage when listening to speakers verses listening via headphones for one simple reason; with speakers your ears hear both sound sources (left and right) but with a delay from the opposite channel which helps position things. But with headphones your ears only hear one source - left or right channel per ear. That creates a totally different soundstage interpretation by your ears/brain. So I think that the soundstage (from the same recording) as a minimum will always be wider (pulled to the sides away from center) when using headphones due to the lack of time delay which one would heard from the other channel if listening to speakers which would move images potentially more into the center of the soundstage. At least that's my current understanding.

     

[Gutbucket]

My current understanding is that you will *never* get the same soundstage when listening to speakers verses listening via headphones..

Of course, there are a few exceptions to every rule.  Below are links to a few sophisticated 'phones that attempt to break that rule. 

I can attest that the 1st linked below does so absolutely convincingly.  It is on my short list of monitoring tools which I'm seriously considering, even though it is rather pricey.  If you ever get a chance to do a demo (which requires personal calibration to your ears in the room/monitoring system to be emulated) do not pass it up as it is down-right astonishing to hear. 

Both the Smyth and the Beyer system are complex DSP driven systems which use binaural techniques to reproduce the speaker HRTF cross-talk you mention. The Beyer system uses averaged HRTF responses and simulated playback rooms.  The Smyth system measures your personal HRTF in a specific playback room.

The AKG1000s were more like speakers you wore on your head in front of your ears, which allowed for some leakage around the head.  A simple mechanical approach that sounded nice, but not nearly as convincing as the DSP based systems.

http://www.smyth-research.com/

http://www.beyerdynamic-usa.com/en/broadcast-studio-video-production/products/headphonesheadsets/headzone-pro1.html

http://www.akg.com/site/products/powerslave,id,249,pid,249,nodeid,2,_language,EN.html

[/thread jack]

[kuba e]

gutbucket: Just to clarify, the reverberation limits in the Stereo Zoom aren't refering to the direct-to-reverberant ratio of sound at the microphone position or the total amount ot reverberant sound in the resulting recording, but instead refer to where the reverberant room sound appears in the resulting payback image. It's more of a warning that with some configurations the center might sound more reverberant than clear and direct, or that most of the reveberant sound of the room will seem to be coming mostly from the speaker positins and not spead evenly acrsoss the stage between the speakers.

The reverberation limits, that are mentioned in the Stereo Zoom, are nicely explained in another Michael's document (see chapter 4):

1987 : 82nd AES Convention in London - Preprint 2466
« Unified Theory of Microphone Systems for Stereophonic Sound Recording » by Michael Williams
http://mmad.info/Collected%20Papers/Stereo/2466%20London%201997%20%2841%20pages%29.pdf

[Gutbucket]

Yes, one of his earlier but applicable papers that helps clarify that part of it.

Welcome to the forum.

[Cobiwan]

Threads like this are why I love this place and spend so much time on the forum.
Thanks to you guys for always handing out the knowledge.