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  #1  
Old 03-18-2002, 05:49 PM
RKrizman RKrizman is offline
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Default HD 192 bass-lite? Impedance mismatch?

This issue came up on some other forums. Someone (a well known and respected Los Angeles engineer) was bouncing some drum tracks to PTHD through the 192 (at both 48 and 96) and then mixing them direct-out through a Neve 8078. He noticed that the tracks coming from Protools were bass-light in the bottom octave, compared to playing the audio straight from tape through the Neve. He had bounced the tracks to PT straight from the Studer 2".

A couple of us guessed that perhaps there was an impedance mismatch between the Studer and the PTHD interface which was sucking out the low end. What about that, Digi, any reason to be careful of stuff like this? Is it better to take console direct outs rather than come straight from a tape machine? Any issues we need to know about, or any other reasons that would account for this claimed loss of low end?

Has anybody else noticed any low end anomalies in the new converters?
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Old 03-18-2002, 09:52 PM
Ask Otis Ask Otis is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

I don't know if this is related, but I found that there was deffinitely a "sweet spot" in the levels with respect to bass response. Did you try altering the levels ?
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Old 03-19-2002, 11:08 AM
RKrizman RKrizman is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by Ask Otis:
I don't know if this is related, but I found that there was deffinitely a "sweet spot" in the levels with respect to bass response. Did you try altering the levels ?<HR></BLOCKQUOTE>

There was no level adjustment. Straight from the Studer into the HD converter, calibrated to -18. There is a major rant going on now that Digi failed to put buffers in the converters and thus the impedance problem. It would be nice to hear from Digi about this before another myth gets lodged into the public consciousness.
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Old 03-19-2002, 01:22 PM
major_thom major_thom is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

wait, so most people dont run a pre amp in between the 2" machine and the pro tools interface when converting to digital?
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Old 03-19-2002, 01:52 PM
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AvidCS AvidCS is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

Hi folks,

Here are the 192 specs from our engineering department;

The output impedance of the 192 I/O's discrete class-A DAC buffer is 50 ohms per leg. The DAC is DC servo balanced, and the LF -3dB point is about 0.1 Hz. The frequency response is flat within 0.05 dB to 20Hz, and because of the slow LF rolloff, should be within a fraction of a dB down to below 5 Hz.

The differential input impedance of the ADC's double-balanced input buffers is 12k. The ADC is DC-coupled, although the ADC has a digital hi-pass filter with a -3dB point at 1 Hz.

DigiCS
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Old 03-19-2002, 02:34 PM
Redbeard Redbeard is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

Just to get this out of the way, because I'm SURE it is not the problem. Was he monitoring on the playback head but tracking from the sync head? Was a playback alignment performed on both heads? I only ask because many times when just bouncing tracks, people won't perform a full alignment.

Red
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Old 03-19-2002, 06:22 PM
major_thom major_thom is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

just bumping this up to the top. can anyone tell me if most people dont use pre amps in between their 2" analog and pro tools interface? is it bad to use pre amps when converting from 2" to digital?
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Old 03-20-2002, 12:08 AM
c-tone c-tone is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

I doubt Digi expects us to run transfers from analog machines through a console on the way to a 192 input.

It seems that this problem could be easily tested by using the same setup and running an oscillator through (or printed tones from) the Studer and looking at the output of the 192 while setup to record. Set 1K to 0 VU, and see where 100 and 50 Hz are.

Also, checking phase on all components and cables may be in order.
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Old 03-20-2002, 12:32 AM
slangification slangification is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

Yeah, smacks of a cabling issue.
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Old 03-20-2002, 05:23 AM
dave-G dave-G is offline
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Default Re: HD 192 bass-lite? Impedance mismatch?

Pre-amps between a 2" deck and the 192 would seem unneccessary since both devices ostensibly operate at +4 line-level.

If it turns out that Mixerman's bottom-octave anomaly isn't a result of impedance mismatch or a sync/repro-head misalignment, that would be reason for concern, since it seems to have been a well-concocted test, involving a common application of the system.

-dg
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