By Bruce Hofer, Chairman & Co-Founder, Audio Precision
This has been a busy year. Two software releases, HDMI & Digital Serial support for APx, four new APx models, a new version of High Speed Tester, five tradeshows, a new solutions- focused website, scores of articles in the knowledge base and too many technical seminars to count.
But it's been worth it. Our rate of innovation is equal only to that of our customers, who continue to push audio capability beyond what any of us could have foreseen twenty-five years ago. I'm proud of the contributions Audio Precision has made to those advances and we will continue to be there supporting you for the next breakthrough and the ones after that.
One thing the end of the year does bring is the final days for System One service. As I noted earlier this year, System One has reached the end of a long life, and it's simply no longer cost-effective to service. To help System One users with the transition, Technical Support has gathered every System One reference they could and added it to a new System One Support page (ap.com/products/s1) including the Service Manual and the original System One Training videos, starring a much earlier rev of the four AP founders!
Finally, while we're all working hard here, I asked Tech Support to find a couple of minutes for some seasons greetings fun. Downloads below.
Best wishes and I hope you enjoy the holidays.
New Features of APx500 Measurement Software v2.3
Over the past two months, AP has already introduced the new hardware components of the latest APx release. The Digital Serial I/O and the four channel APx521/526 are both significant achievements, but they shouldn't completely overshadow the new software features that APx v2.3 delivers: Digital Error measurement, HDMI EDID, save-to-disk audio recorder, custom sweeps, Signal Diagnostics, support for more coded audio formats in File Playback, appended sweeps in a sequence, improved UI and numerous other enhancements.
Two of the most interesting new features are the Digital Error Rate measurement and HDMI EDID. We'll look at HDMI in more depth in January. Also in January, we'll look at the possibilities delivered by the new APx LabVIEW driver and new Dolby/dts confidence testers that are included on the revised APx Resource Disc.
Bit-for-bit accuracy with new Digital Error Rate measurement
Any device that just passes through digital streams (bitstream mode) strives to be bit-accurate. However, mistakes can happen. Devices may unintentionally truncate data, introduce dither, scale the signal, or convert the sample rate. These errors may be hard to detect with a traditional THD+N measurement, but they can cause havoc for devices receiving the corrupted data later in the signal path.
An excellent case in point occurred last month, when a team of AP engineers visited the CEA's HDMI PlugFest in San Francisco. An attendee at the PlugFest had a new chip that took SPDIF in and output an HDMI-ready data stream. When we hooked it up to an APx585 with HDMI, it reported a digital error rate over 70%. Deeper investigation proved that the device was accidentally killing the last four bits of the 24 bit word. The engineer was surprised to say the least, but grateful that we'd found the error before the chip was handed over to customers for evaluation.
Fig 1: Average error rate of a device unintentionally truncating the last four bits of a 24-bit word.
APx calculates the Digital Error rate by taking a pseudo-random waveform with values uniformly distributed between plus and minus full scale and performing a bit-for-bit comparison between the data received from the device under test and the original file. A device passing the data correctly will have a 0% error rate.
Fig 2: Slice of the Bittest Random signal used to exercise every bit in a word to determine bit-for-bit accuracy.
The digital data analyzer in 2700 Series and ATS-2 have both provided similar functionality for years. However, one key innovation of the APx Digital Error Rate measurement is the ability to test the encoded audio streams typical in multichannel audio. The APx's technique allows both lossless and (through some clever signal manipulation) lossy streams to be tested, crucial for evaluating whether an HDMI source can truly handle all the different formats that may be requested by an HDMI sink.
The Digital Error measurement can also be used to test bit-for-bit accuracy over time (to over 10,000 minutes if you need it!), a useful feature if you suspect intermittent errors that would otherwise be hard to reproduce. Available views of the data include Average Error Rate, Instantaneous Error Rate, Total Errors and Cumulative Errors over time.
New digital diagnosis tools have also been added for Signal Path Setup. The walking bit waveforms are most useful when viewed in a data bits display. The walking pattern makes it easy to observe "stuck" bits in digital hardware. The walking bits waveforms will also show no errors when passed through a bit-accurate system, though the Random Bittest waveform is more appropriate for more detailed diagnostics as the bits of the word are exercised more thoroughly.
Fig 3: Walking ones progressing from most significant bit to least significant bit.
AP continues to add new Knowledge Base articles, some specific to AP gear and some more general audio test topics. A recent post from Director of Tech Support Joe Begin looks at making distortion measurements when limited to test times under 1/5 second.
We need to conduct distortion tests of power amplifiers during design and development, before heat sinks have been installed. Without heat sinks, the DUT can only withstand a duty cycle of approximately 0.2 seconds on and 0.8 seconds off. In addition, the amplifiers may have an onboard DSP, so the test needs to account for a delay in the DUT. How can we do this test with the AP2700?
To conduct these measurements with a very short duty cycle, we recommend using the Arbitrary Waveform generator in the AP2700 with the FFT spectrum analyzer.
First, we construct a waveform file that has the appropriate duty cycle. Please see the attached macro file “Burst THD_Mod1.apb” and the accompanying test file “Burst THD_Mod1.at27”. For this sample test, we have set the output sample rate at 16,384 Hz. Given the generator buffer length of 16,384 samples, this provides a generator buffer 1.0 seconds long. In this sample project, the waveform loaded into the generator buffer consists of six cycles of a 32.0 Hz sine wave with the rest of the 1.0 second buffer padded with zeroes. This results in a duty cycle of 187.5 msec on and 812.5 msec off (see Figure 1). In the example, the Auto On feature of the Analog Generator is used. As a result, the system has plenty of time to turn the generator off, preventing the DUT from being exposed to successive bursts of signal.
Figure 1. Plot of the waveform used in the Burst THD_Mod1.at27 test
In the test file, note that the Input sample rate is set to HIRes A/D @65536 and the FFT size is 4096. As a result, the FFT transform buffer length is 62.5 msec, or exactly 2.0 cycles of the 32 Hz sine wave. The frequency of 32 Hz was chosen as a synchronous frequency - an integer number of cycles fits exactly into the FFT buffer, allowing a leakage-free FFT without windowing.
Note also that the FFT start time has been set to 93.75 msec. This allows for a delay time in the DUT. This value was arbitrarily chosen as being one half the length of the signal-on time.
In the test file, the sweep table for the FFT spectrum has been set up to have 10 points (the fundamental frequency and the first 9 harmonics). The macro runs the test and then computes the THD as the RSS (root sum of squares) summation of the 9 harmonics relative to the fundamental. The THD number is shown below the spectrum graph when the macro finishes (Figure 2).
Figure 2. Spectrum from the Burst THD_Mod1.at27 test and the calculated THD.
A second set of files named “Burst THD_Mod2.apb” and the accompanying test file “Burst THD_Mod2.at27”, provide the same measurement, but this time with a signal at a frequency of 1024 Hz.
For the IMD measurement, please see the attached macro file “Burst IMD.apb” and the accompanying test file “Burst IMD.at27”. This test uses the same principles as the THD measurement above. However, in this case the IMD test uses sine waves with frequencies of 18 and 20 kHz. To accommodate these higher frequencies, a higher sample rate must be used in the signal generator. Unfortunately, the higher the sample rate is, the shorter the time length of the generator waveform buffer. In this case, we selected a sample rate of 51.2 kHz. This provides nice whole number frequencies that fit within the generator and FFT transform buffers, and it provides a generator buffer length of 320 msec.
Figure 3 show the waveform used and Figure 4 shows the spectrum.
Figure 3. Signal used for Burst IMD measurement in test “Burst IMD.at27”
Figure 4. Spectrum from burst IMD measurement in test “Burst IMD.at27”
Based on the measured FFT spectrum, this macro computes the second and third order IMD difference products and their RSS sum. It then calculates the Total RMS IMD as the ratio of the above distortion products to the overall RMS level from 50 Hz to 20 kHz. The results are listed in the Immediate window of the macro panel (Figure 5).
Figure 5. Results of IMD calculations in macro “Burst IMD.apb”
Jingle Bells for AP
The 2700 Series has often been called a swiss army knife thanks to its versatility. But has anyone programmed Jingle Bells in AP Basic? They have now.
In contrast, while APx does have a wide-open API, a lot of the need for custom generator scripting has been superseded by APx's File Playback capability, which can use any WAV file up to 32 Megasamples in length as a stimulus...
Which is better? It really depends on the application. For Christmas parties, either one meets spec.
Final Reminder: End of System One Service
Wednesday, December 31, 2008 is the last day to order System One service. After that date, no service orders for System One will be accepted. Tech Support will also be only able to offer limited System One support. AP has created the System One Archive Page with all the reference materials we have relating to System One. Please visit http://ap.com/products/s1
AP Holiday Closure
AP will be closed for the holidays for the week of Dec 22 to Dec 26.