Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
It's a new year, and we hope that 2013 treats you and your projects well. AP is looking forward to a terrific 12 months, with product releases that we are sure will help you to get even more done with our analyzers.
We are seeing more opportunities arise in 2013 as audio evolves in new classes of products, many of which were unimaginable not long ago. It is an exciting time, and AP is thrilled to be a key part of your success.
Aligning Analog Tape Decks with APx
Even in a digital world, some recording studios and archives still use analog tape. These facilities must periodically align their analog tape decks to achieve optimal performance. This article explores multitone measurement techniques, as well as the new APx Wow and Flutter and Repeat Measurements Utilities that together make AP’s APx analyzers a rapid and precise solution for analog tape. We’re assuming some familiarity with standard deck alignment in the discussion that follows, but even if analog tape isn’t your thing, it can be eye-opening to learn what it took to routinely maintain audio recorder performance before digital came along.
by Adam Liberman, AP Technical Support Engineer
Traditional Playback Alignment
Traditionally, adjustment of playback azimuth, frequency response, and level has been achieved using expensive alignment tapes. These tapes, such as the MRL Multifrequency Calibration Tape, contain a series of tones of varying frequencies, separated by voice announcements. While AP’s 2700 Series instruments are able to ignore the voice and plot the frequencies automatically, it is still a time consuming process to play the entire tape, and typically sections must be replayed as adjustments are made.
An Imprecise Reference
Although the standard reference tapes are precisely recorded, their precision starts to deteriorate from the first time they are used. Factors such as magnetization, stretch, surface wear, edge damage, and self-erasure from bending over guides cause changes in the level and stability of the recorded tones, especially at the higher frequencies. Although a typical regimen may specify replacing the tape after 30 or so plays, the actual rate of deterioration depends on the condition of the decks it is played on, and the only way to measure this is to periodically compare it to yet another reference tape that is kept for that purpose.
Multitone signals contains many tones at once, and can be generated and analyzed using the APx500 Multitone measurement. For tape alignment, the standard “APx 32-tone” signal (1/3 octave intervals) works well. Although no standard reference tape with multitone may be purchased, you can make your own using a tape deck in excellent condition after it has had playback precisely aligned with the reference tape, followed by record alignment with multitone. For tapes running at 7.5 ips, the multitone recording should be made at 10 dB below reference fluxivity to avoid the possibility of high frequency saturation.
Fig 1 Multitone stimulus signal, comprised of 32 individual tones.
Fig 2 Frequency response graph using the multitone stimulus.
Using a multitone tape for playback alignment has numerous advantages:
Note that we haven’t completely eliminated the need to own a first generation standard reference tape. It’s still the reference standard that we base everything on. But by using it only on rare occasions to prepare our multitone tapes, we keep it accurate at the same time that we improve the speed and accuracy of our routine deck alignment.
The second generation multitone tapes, if made very carefully, will be much more accurate in routine alignment service than a worn first generation reference tape, as they can affordably be discarded and replaced after just a few or even one usage. You can, in fact, record an entire reel of multitone, but use caution here: you need to monitor playback response the entire time, because unless the deck is in excellent mechanical condition, the heads stay spotlessly clean, and bias is absolutely stable, high frequency response and azimuth position may not remain consistent from the beginning to the end of the reel.
Fig 3 Record and playback adjustments on an Ampex 440 analog tape deck.
After playback alignment has been done using the multitone tape, it’s time for record alignment. The first step is normally to “overbias” the recorder—record a tone, usually 10 kHz, and then increase the record bias until the playback level peaks and then drops by a specified amount. The amount depends on the tape type and the head gap length, and is specified by the tape manufacturer. Once bias is set, the multitone signal from the APx analyzer is used to adjust record azimuth and record equalization.
Wow and Flutter
The new APx Wow and Flutter Measurement Utility lets you precisely measure wow and flutter on an APx analyzer without requiring any additional hardware. If you have a high quality studio tape deck, you may find that its wow and flutter is lower than the residual on a standard speed reference tape. In this case, you’ll get best results by recording a 3 kHz reference tone and then measuring wow and flutter while playing back the tape.
Fig 4 The APx Wow & Flutter Measurement Utility.
Bridging Both Worlds
With the new APx Wow & Flutter and Repeat Measurement utilities complementing its extensive multitone and FFT capabilities, a single APx500 analyzer can now serve the maintenance needs of studios and archives with both vintage analog and the latest digital gear.
Sound Advice: AP Announces PDM Line Driver
AP is proud to unveil a useful new complement to our PDM (Pulse Density Modulation) Option, the PDM Line Driver.
The AP PDM Line Driver solves a key problem encountered when testing PDM MEMS (Micro Electro-Mechanical Systems) microphones: the distance between the DUT and the analyzer itself.
PDM data is one-bit oversampled audio, most commonly employing a oversampling factor of 64. This means that in order to represent the performance of a PCM audio system with a 48kHz sample rate, many digital MEMS microphones employ a sample rate of 48kHz x 64 = 3.072MHz. The low-power, current-limited design of MEMS devices means that this high-frequency signal can suffer severe degradation due to cable capacitance in many test bench configurations, especially where the DUT must be a substantial distance - 1 meter or more - from the analyzer.
Testing microphones in an anechoic chamber is just such a scenario. All noise-generating equipment (PCs, analyzers) must remain outside the chamber, requiring substantial cable runs from the inside.
AP PDM Line Driver with cables
The Audio Precision PDM Line Driver provides a buffer for the PDM audio data, allowing cable lengths of up to 45ft (13.7m) to be used between DUT and analyzer with no loss or alteration of the digital signal.
Maximum cable length vs. PDM clock frequency
The AP PDM Line Driver will be available in February, 2013. Contact your AP Partner for ordering information.
Test Results: AP News & Events
© 2012 Audio Precision, Inc.