User's Manual

66
User’s Guide ADI-2 DAC v2.2 © RME
31.15 Bit Test
A bit test is used to check the playback path for unwanted changes in the playback data.
Playback software can cut bits, add dither, or change the level - without these changes
becoming noticed easily. A poorly programmed driver can manipulate bits, and a playback
hardware could be both badly designed and defective (hanging bits, swapped bits). Even such
features as proper channel assignment, left/right synchronicity and polarity can be tested by a
well-made bit test.
With a bit test, such errors can be detected and - more importantly - excluded.
How does it work?
Most bit tests take some time and are loud and unpleasant when playing through headphones
or speakers. RME uses a unique bit pattern, with defined levels and pauses. This consists of
only 400 samples (<10 ms), and sounds like a dull, medium-loud click - harmless for ears and
equipment. The short, but efficient test sequence allows to check for the following changes and
errors:
Level changes, equalization, dynamic processing, polarity, channel swapping, sample offset,
hanging or twisted bits, dither, bit reduction.
The signal reaches the ADI-2 DAC via USB, AES or
SPDIF/ADAT. The unit has three continuously running check
circuits. If the test signal is detected correctly, the device's
display shows a message: Bit Test 16 bits, 24 bits or 32 bits
passed, depending on the detected signal. If the
transmission path is not bit-transparent or bit-accurate, the
signal only minimally changed, the message is not shown,
the bit test failed. There is no error notice.
RME provides several audio files as free download: 44.1, 96 and 192 kHz in 16 bit, 24 bit and
32 bit. These files in WAV format can be played easily on Windows, Mac OS X and Linux. For
ease of use (looping, players with fade in/out), the files contain the bit pattern several times.
Runtime is about 4 seconds.
Download:
http://www.rme-audio.de/download/bit_test_wavs.zip
The Zip archive contains:
441_16_adi2pro_bittest.wav 441_24_adi2pro_bittest.wav 441_32_adi2pro_bittest.wav
48_16_adi2pro_bittest.wav 48_24_adi2pro_bittest.wav 48_32_adi2pro_bittest.wav
96_16_adi2pro_bittest.wav 96_24_adi2pro_bittest.wav 96_32_adi2pro_bittest.wav
192_16_adi2pro_bittest.wav 192_24_adi2pro_bittest.wav 192_32_adi2pro_bittest.wav
Theoretically, the use of the 32 bit file is sufficient. If the lower bits on the transmission path are
simply truncated, the corresponding message appears with the respectively recognized bit
resolution, i.e. 24 or 16 bits.
Notes
:
¾ iOS, AES, SPDIF and ADAT are limited to 24 bit.
¾ Some players in Mac OS X offer a Direct Mode, using 32 bit integer in non-mixable format.
The 32 bit test might still fail. At this time only HQPlayer 3.20 is known to pass.
¾ SPDIF/ADAT (AES) are checked behind clocking. Therefore the unit needs to be synchro-
nized correctly to the digital input signal.