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RE: Cable article link

Posted by jrlaudio (P) on October 20, 2013 at 20:06:59

In Reply to: RE: Cable article link posted by Tony Lauck on October 18, 2013 at 17:36:26:

I agree that amplifiers can pass these ranges within their "power bandwidth". However, not always so in terms frequency response or phase response stability. Additionally in other electronics these signals can be inherently unstable and cause adverse interaction when seen strictly in terms of amplitude and phase.

Also these frequencies, if recorded at high enough levels, can have an adverse effect upon accurate metering, whether dBm or dBFS scaled. They can also impact the performance of the AD/DA converters, regardless of bit depth or sample rates. Case in point, at one session on an SSL console we were seeing a substantial signal on the console channel for an electric guitar amp. Unknown to us a bad tube was self-oscillating ultra-phonically and the microphone was passing this signal to the SSL. Of course the SSL "super analogue" circuits were passing this along to the HD I/O of Pro-Tools. It was higher than the guitar level being played through the amp. The meter stayed fixed at one level even when the guitar was being played. However on the Pro-Tools HDX interface this was not showing on the output of the converter, but was showing on the input. The recorded signal was terrible, due to massive error correction going on at the a/d converter or possibly simple overload. All the while what we were hearing sounded normal from the "super analogue" output from the console through our near-fields and the amplifier itself sounded fine acoustically. Switching in a 22kHz low-pass filter on the SSL is what allowed us to discover the problem. Obviously we couldn't hear it. We rolled through the tubes in the amp, found the offending tube, replaced it and moved on. Problem solved.

Having these frequencies in a recoding may also lead to instability issues and wasted power in playback of the end product, especially in consumer level playback equipment. And this can certainly lead to IM artifacts within the "normal" audio band. Especially in tweeters that cannot produce these frequencies. In certain cases this can actually lead to unnecessary and completely avoidable heating and subsequent failure of those drivers.

The reason why the response of professional amplifiers and microphones is extended out to these frequency extremes is not to capture or reproduce them, it is to extend any instability out to frequencies beyond the normal working passband therefore reducing band edge interaction within these devices themselves. However having wider power bandwidth on an amplifier should never infer stable performance above the normal stable 20kHz frequency response. It only assures stability at that limit.

These and other reasons are generally why these frequencies are filtered and not because we feel they do not exist, it is simply good engineering practice. Even this statement is not absolute since one can extend this higher, based upon the requirement of the recording. One case being ultrasonic modulation of audio frequencies that occur in pipe organs that effect timbre of certain stop combinations. However these conditions are rare and specific.

Edits: 10/20/13 10/20/13 10/20/13 10/20/13

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