Conference napalm::commusic_v1

Noise is usually measured in reference to a standard output signal, usually the
loudest music you can play.  It's probably best to use an oscilloscope to
measure it.  The equation is:

DB = 20 X LOG (noise level / signal level)

For instance, if the noise level is 0.01 volts and the signal level is 1 volt,
the signal to noise would be 

DB = 20 X LOG (0.01 / 1 ) = 20 X -2 = - 40 DB

Cassette tapes are usually around -60 DB
CD's are between -90 and -110 DB

You can easily hear noise that is between -40 and -50 DB below the signal.

    You need a scope because a typical tool-box AC voltmeter can not
    measure the kind of frequencies that might be present.  The noise is
    essentially white noise.  Also scopes are typically much more
    sensitive.
    
    I took an LP turntable (remember them) back to the dealer because I
    heard a swishing noise that turned out to come from static electricity
    generated in the nylon bearing, radiated by the aluminum platter, and
    picked up by the cartridge.  The guy at the store got out his test
    record and S/N measuring device and we measured this noise.  It turned
    out to be entirely within spec.  In a quiet room, it is amazing how
    soft of a sound you can hear.

    When I went to master last week and listened to the D70 over the DAT I
    could clearly hear noise.  The meter on the DAT went down to about -70
    dB or so.  The D70 went up to a max of about -30 dB, covering a 40 dB
    span.  Even though I could hear the noise, it didn't register on the
    DAT's meter.  So, I figure that the D70 has about a 50 dB S/N ratio
    or so.  You might try a similar experiment with your box.  I don't know
    if the manual has a spec on S/N ratio for the D70.
    
    One other point, when at home I didn't notice the D70 noise so much. 
    Two reasons.  The first is that in the home I run the sound through my
    home stereo system.  The noise of the home stereo with nothing hooked
    up is louder than the noise generated by the D70.  Second, when I make 
    a tape the noise is typically much less than tape hiss.  It's only when I 
    go straight to DAT that I really notice the noise because only then do I
    have a sound system that gives me dead silence.  This is a pro-DAT recorder,
    too.  I don't know if lesser DAT recorders give you any noise.
    
    Steve

    When using a scope to measure the noise on a keyboard, how can you tell 
    what part of the signal is noise?  Would I have to setup a patch that
    just generates a sine wave so that the noise signal is obvious?

    Calvin    

    It depends on whether there is any noise when you are *not*
    pressing a key.  If there is, then you measure it by itself.
    Then you sound the note and measure total amplitude of note
    and noise together.
    
    If the noise is only there during a note, then it is probably
    quantization noise in the digital synthesis process.

    Thanks for the tip on measuring s/n ratio on synths.
    
    Calvin