The Commish’s Blog

The Commish’s Blog

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It’s about the journey, not the destination.

AV 9000 Standard Checklist Item Under Test:  

Prepare document report, certifying that the product, performance, and practices are in compliance, and noting any exceptions below. Distribute accordingly.

Testing Reasoning: 

Whether you are commissioning a system as a third party, or as part of the quality assurance team in your company to make sure your client has everything they paid for, the integration process can be more important that the testing itself.  It certainly is where profits are made.  If the way the systems are being installed are not in harmony with the commissioning (the end game), finishing projects is going to be painful.  If, however, the way the systems are being installed is done with the end in mind, the checklists are an afterthought.  They just confirm that your great work is…great.  Systems integration should be performed while focusing on the client instead of rushing through the process and relying on a series of tests to catch the issues.

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Which should rule DSP meters?

AV 9000 Standard Checklist Item Under Test: 

For audio conference systems, confirm microphone input gain settings are such that the "standard talker" (60 dB SPA at 1 m, IEC 60268-16), positioned at each talker position in the room, produces a nominal “0 dB” level at the input of the mixer bus of the audio conference DSP device. If there is local reinforcement ("mix-minus"), AGC and ALC may need to be restricted. Record test results as pass/fail. Record level across analog telephone line. Inspect DSP mixer telephone line levels, both transmit and receive, when normal speech is encountered in the room.

Testing Reasoning: 

In order to provide optimal signal to noise ratio on microphone channels, while still allowing for adequate headroom, the AV 9000 standard suggests setting all audio levels (microphones, conference transmit, conference receive,  etc.) in a DSP mixer to “0 dB”.  The reason “0 dB” is in quotations is because sometimes it is a solid yellow, sometimes it is actually 0 dB read on a meter, and sometimes it might be -20 dBFS.  As common as “0 dB” (typically 0 dBu) is in today’s DSP mixers, I think dBFS may be more informative and useful to the site file programmer.

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Understanding What the DSP Meter Shows

AV 9000 Checklist Item Under Test:

For audio conference systems, adjust microphone input gain so as to demonstrate that "standard talker" (60 dB SPL at 1 m), positioned at each talker position in the room, produces a “0 dB” level at the input of the mixer bus of the audio conference DSP meter. If there is local reinforcement ("mix-minus"), AGC and ALC may need to be restricted. Record test results as pass/fail. Record level across analog telephone line. Inspect DSP mixer telephone line levels, both transmit and receive, when normal speech is encountered in the room.

Test Reasoning:

Several DSP manufacturers deploy different types of gain controls for their microphone inputs.  One might be called “Course Gain” or “Input Type” and adjusts the level while the signal is still in its analog form.  The other might be called “Fine Gain” or “Level” and adjusts the level after the signal has been converted to digital.  There is a rule of thumb that says most of the gain should happen in the analog stage, and the digital gain should be used minimally.  Understanding why this rule of thumb was created will help the operator set the system gain structure properly.  If the analog gain is set too high, and the signal distorts, attempting to attenuate the level at the digital gain stage will only decrease the level of that distorted signal.  It does nothing to remove the distortion.  To make matters worse, the DSP meters may show the level as the “perfect 0 dB” signal in the digital realm, even though the signal hit the rails at the analog realm.

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