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9.1
Testing CD Players
CD2, a Lindos Test CD, available from Lindos, or as a download at
www.lindos.co.uk (use a ‘Music’ CD-R when making a test CD – see later
comments), has been designed to facilitate the rapid sequence testing of CD
players. Unlike most test CD’s, this one has all levels referenced to Alignment
Level, which means that, unlike most test CD’s, it does not immediately burn
out your speakers or deafen you with unrealistic levels! By measuring noise
relative to Alignment Level, meaningful figures are obtained, though both low
and high level bands up to FS are provided for further distortion checks. A
variety of sequences is provided, along with tones and silence (with or without
dither or noise-shaping).
Just connect the MS10 using appropriate Lindos leads, and press SEQ. Then
play Track 13 which will generate SEQ3 at –18dB FS alignment level, and wait
for the sequence to finish (by listening, or by looking for the next track to
commence). Track 4 has been allocated deliberately as Mute, so that there is
no rush to pause the player. When playing other tracks it may be necessary to
pause fairly quickly, or use the program facility on the player, to avoid FSKtriggering by the next sequence.
A typical results page (see Fig 1) should show a Noise level of –65dB or less.
All sequence tracks on CD2 use noise-shaping rather than other forms of
dither to achieve the lowest residual noise floor, allowing very good players to
be distinguished. The +18dB Dist Residue should be reasonably low (-50dB or
less). The ‘Test Level’ figures in the results, show the deviation of output level
from the assumed UniSon standard. Consumer players typically have a
specified Full-Scale output level of 2V FS (+8.23dBu) which corresponds to
+14.23dB relative to the Unison alignment level. An Alignment Level tone
18dB below this therefore gives rise to a Test Level of -3.68dB (14.32-18).
All tracks are duplicated at an alternative Alignment level of -15dB FS. While –
18dB FS is the main recommended EBU Alignment Level for studio
recordings (other EBU recommended levels are –12 and –24), there is a case
for using Track 3 (-15dB FS), rather than 13, especially when testing
consumer CD players, for two reasons. Firstly, it results in an output level
close to the UniSon standard (-6dBu) on typical consumer CD players.
Secondly, there is actually no recommended headroom figure for CDs and in
recent years recorded levels have been pushed ever higher in a rather stupid
and futile attempt to make ‘pop’ recordings in particular sound louder than the
competition! While early CDs tended towards having peak levels 18dB above
‘typical’ level, the worst recent pop recordings spend most of the time within
6dB of FS, so 15dB is currently a good working figure to assume. Thirdly,
most items of consumer equipment; camcorders, set top boxes, and PC cards
especially, fall far short of true 16-bit performance, so trading a little headroom
for less noise, by assuming an alignment level 15dB below FS for all
‘consumer’ equipment is perhaps a good idea. If test files aligned at –15dB FS
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