The 5-minute Guide To Chip-level Audio Test_
Audio test procedures need to be quick and easy, especially in the R&D department - it doesn't make sense to hold up progress with detailed, over-complex test routines. But in these days of multi-channel audio for high-definition broadcast and surround sound hi-fi, simply testing the outputs of a developing AV device isn't really exhaustive enough. In R&D and in the test bay, you want to be testing at chip level; if an individual circuit board component is unreliable, measuring audio at the device's output will do nothing to help you trace the fault.
Assuming you have access to such a test device, you then need the right kind of cabling and connections to talk to the board-level components you'd like to test. You'll be hard pushed to find many quarter-inch jack plugs at this level of operation; instead special serial cables are used, sometimes packaged as one cable with many breakout connectors at one end, and sometimes supplied as multiple cables, terminating in board-level connectors.
If you have all of these things, you then have to consider what language is being spoken by the chips and board connectors. Nearly all test and measurement equipment with serial interface capabilities can receive and retransmit i2S (properly IIS or Inter-IC Sound), which is the nearest thing board-level components have to a common format. Audio and clock data is always transmitted separately by I2S, hence the multiplicity of cables.
However, not all manufacturers' serial protocols are the same, and some are bespoke or proprietary. An analyzer capable of receiving and transmitting many different combinations of clock rates and word lengths is a must, as is careful study of the serial timing diagram supplied by most circuit board manufacturers, which will usually enable you to set up the analyzer so that it can correctly talk to the chips under test. Some analyzers, such as those manufactured by Audio Precision, guide the tester through the process of setting up the serial digital interface correctly, prompting for the entry of the values from the board manufacturer's timing diagram and giving visual feedback in the form of an on-screen 'active timing diagram' when setup is complete. If you have a test oscilloscope, you can carry out a three-way check that you have the correct timing information for the chips you're trying to test: if you've done it correctly, the timing info on the diagram, on the scope, and in the analyzer's active timing diagram (if it has one) should match.
Making It Easier
That's all you really need for board-level audio test, but some analyzer features can make life easier. Multi-channel analyzers allow faster testing, as many connectors on the board can be tested in one pass. As already mentioned, an external scope provides a great way to good cross-check, and if your analyzer offers a buffered output on its serial interface connectors, you can connect the scope to the serial interface directly. Similarly, testing the clock input and output of sample-rate converters requires two clock sources independent of the SRC itself, so an analyzer with two built-in clocks is useful, although these are rare. Finally, once you've set everything up for successful testing, it's handy if your analyzer can save all the data and parameters you've just entered as a recallable setup memory, so you don't have to go through the same procedure again next time you're testing components on a board of the same type.
Top Tips for Testing Chips
- The instructions for connecting to demo boards are often incomplete or incorrect. Make sure you have the latest versions.
- The unmarked side of double-row pin headers is often ground, but it's not always safe to assume that.
- Observe static protection measures to prevent failures (yes, you really can fry those chips just like that…)
- Don't use excessively long test cables.
- Test all the sample rates and modes the device is supposed to work at. Just because it works in one mode doesn't mean the others will be OK.
Daniel Knighten has been with Audio Precision since 2000. Knighten spent many years at AP troubleshooting customers’ audio test issues as manager of the technical support department before moving on to serve as director of products. He was recently promoted to vice president of sales.