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From: jchristy@hpspcoi.HP.COM (Jim Christy)
Newsgroups: sci.electronics
Subject: 0 dB reference: Does it mean alot?
Message-ID: <490004@hpspcoi.HP.COM>
Date: 24 Aug 90 01:53:18 GMT
Organization: HP PCG, Sunnyvale CA
Lines: 39


Does 0 dB mean anything across audio compnent families as far as the 
signal voltage is concerned?  To try and answer this I did the following:

1. Played the CBS Test Disc (reference CD) on track 1 which is marked
1KHz 0 dB reference signal.  THis was played on my Sony CDP 520ES CD player.
I measured the signal output with a scope at the back of the player and
found it to be 2.8 volts peak to peak.

2.  Recorded the same signal with my Nakamichi 480 tape deck, after
adjusting VU meters to read 0 dB.  When played back, the signal voltage
at the back of the deck was 1.6 volts pp.

3.  Recoreded the same signal with my Mitsubishi VCR on the audio tracks,
after adjusting the VU meters to 0 dB.  When played back the signal read
0.8 volts pp.

Summarizing:

0 dB from CD:  2.8v pp.
0 dB from tape deck: 1.6v pp.
0 dB from VCR:       0.8v pp.

Is this really what one should expect?  It seems the CD player output is
too high (and it has no output level control).  Both the VCR and tape deck
VU controls had to be set very low to prevent pegging the meters.  On the 
other hand, output levels from the tape media do not agree either!  When
designing a circuit to feed a stereo receiver or amp via the TAPE IN jacks,
what is a good 0 dB reference point?  

Any help or ideas would be appreciated.


Jim H. Christy
Hewlett-Packard, PCG
974 E. Arques Ave. MS 72UT
Sunnyvale, CA 94086

jchristy@hpspcoi.hp.com