By Bruce Hofer, Chairman & Co-Founder, Audio Precision
The first half of the year has gone by extremely quickly. In February we released a LabVIEW driver for the 2700, in March we released APx v2.0 , and I'm pleased to report that < AP2700 v3.30 / ATS v1.60 and our USB Adapter for 2700/ ATS are both on schedule for release August 15 and September 7 respectively. We've invested a lot of effort this half supporting customer requests, and it shows with some of the largest order totals the company has ever seen. Thank you.
I'd also like to thank you all of you who participated in our v3.30 / v1.60 beta program for AP2700 v3.30 and ATS v1.60 over the past month. Feedback like this, as well the input from customers out in the field, is what allows us to continue to lead the market in audio test equipment.
I hope your summer is going well (for those in the northern hemisphere) and I look forward to seeing you at IBC Amsterdam and AES New York.
Revamped Sample Tests
One of the most valuable resources available for the 2700 and ATS-2 is the set of Sample Tests included on the Resource Disc. The new v3.30 and v1.60 software releases include completely updated sample tests.
The Resource Disc has over 130 tests. All the standard audio tests, such as Frequency Response, Phase, THD+N and FFTs are included, with different versions for each domain you might need: A-to-A, A-to-D, D-to-A, and D-to-D. There are also several Digital Interface tests with differently configured Eye patterns as well as some useful Macros and tests for the PSIA module.
New to this release are 75 pages of extensive documentation describing the purpose of each test, how the stimulus is set up and how to make the measurement. This documentation makes it much easier to find the right test template and customize it for your situation.
The revamped Resource Disc will be available for download from the software section of the website August 15.
Beta update We've opened up the access so now any registered AP user can download the beta versions of the new v3.30 and v1.60 software releases. Download today at ap.com/beta. The final release of the software (with documentation and supporting resources) will be available for download August 15.
AP Applications Engineer Eric Schultheis continues where April's article on jitter left off.
Measuring jitter with a 2722 Audio Analyzer
As April's Audio.TST discussed, jitter is the variation in time of the derived clock signal from nominal. Jitter can be introduced into a digital audio signal during the sampling process, and/or by the digital interface. The three types of jitter are “sampling jitter” (induced by an ADC's imperfect clock), “sync jitter” (induced by a receiver's VCO imperfection), and “interface jitter”, where cable reactance and/or improper impedance cause a loss of high frequency, thus smearing the clock's pulse transitions. A digital audio system relies on sample points at specific locations in time. Jitter results in distortion of the reproduced audio stream within the digital stream as sample points are shifted forward and backward in time.
The 2722 Series audio analyzer > offers three methods of jitter detection using the Quick Launch macro Intervu. Intervu analyzes the AES/EBU or consumer serial digital interface input signal via an 8-bit A/D converter with an 80 MHz sample rate, providing an analysis capability with over 30 MHz bandwidth. Intervu acquires 1.5 M samples of the interface signal into a buffer, resulting in about 19.66 milliseconds of data.
The Preamble jitter detection selection uses the average rate of the trailing edge of the first three-UI-wide pulse in each preamble as the stable clock reference. Each actual transition at the trailing edge of the first three-UI-wide preamble pulse is then compared to that reference (average value) to obtain jitter values for display as jitter waveform, histogram of jitter, or FFT spectrum analysis of jitter. The three-UI pulse in a preamble is the most robust portion of the digital interface signal, since it is least affected by reduced bandwidth in the cable or system. Therefore, jitter measurements made with the Preamble jitter detection selection tend to be measurements of the intrinsic jitter in the transmitting device clock and are relatively unaffected by data jitter caused by reduced bandwidth. Since this derived reference clock rate is low (twice the audio sample rate), the effective jitter measurement bandwidth equals the audio sample rate when Preamble is selected.
The Stable Bits jitter detection selection derives the stable reference clock at 1/4 the actual cell (bit) rate, synchronized to the beginning transition of the preamble. The serial signal consists of 32 cells (bits) per subframe and two subframes (left and right channels) per frame. The frame rate is equal to the sample rate of the audio. Thus, there are 64 cells (bits) in a complete frame and the cell rate is 64 times the audio sample rate. The first four cells of each subframe are the preamble. The preamble always starts with a three UI (1 1/2 cell) wide pulse followed by sequences of one UI, two UI, and three UI pulses which are different among the three possible preambles. There is no cell transition time within the preamble which is common to all three preambles. The highest rate at which transitions can be guaranteed to occur regularly is at 1/4 the cell rate, which includes the beginning and end of each preamble but no transitions within the preamble. This rate is 16 times the audio sample rate, so the effective jitter measurement bandwidth is eight times the audio sample rate (384 kHz at a 48 kHz sample rate).
The All Bits jitter detection selection derives the stable reference clock at the actual cell (bit) rate. Since there are 64 cells per frame and the frame rate is the audio sample rate, the reference clock is at 64 times the sample rate and the effective jitter measurement bandwidth is 32 times the audio sample rate (1.536 MHz at a 48 kHz sample rate). Since the preamble of each sub-frame will not have transitions at every cell boundary due to its three-UI-wide pulses (violations of bi-phase coding), the DSP interpolates where transitions would have occurred if the preamble did not violate bi-phase coding.
Using the Jitter Detector is another method of measuring jitter when single readings are required (as opposed to a graphic display) for AES 3 or IEC 60958 type specifications or when measuring sample rates above 48 kHz. The Jitter Detector is visible on the Digital I/O panel's maximized view. Also, while jitter accuracy is not defined within the Intervu macros, it is specified for Jitter Measurement (via the Jitter Detector) as ± (10% + 1.0 ns), peak calibrated.
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Competition: Test Bench of the Year Every engineer should be proud of his test bench. Whether it's stacked to the ceiling with every piece of gear ever made, perfectly squared away or looks like a cyclone hit it (but it works!), your bench says something about who you are and how you operate. AP invites you to submit a photo for the first Test Bench of the Year competition. Winners in each category will get their photos posted, overall winner gets an iPod Nano. Extra points for prominent AP gear.
Test Bench of the Year Competition Categories
Send images (2MB max) to email@example.com. Please specify if you want your company name posted with the photo or not.
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