Here are my guesses, from most prominent to most subtle:
Almost everything in the analog realm introduces:
low pass filtering
harmonic distortion (introduction of harmonics of the input
frequencies - it will take each harmonic of the input sound and put
new harmonics above that). This can be done in csound as waveshaping
(though there is the issue of bandlimiting to prevent foldover).
intermodulation (basically every time two frequencies are present,
some amount of quiet ring modulation between those frequencies will be
introduced). This is easy as "asig = a1 + a2 +
(a1/0dbfs)*(a2/0dbfs)*kintermod" where kintermod should be a very
small number (maybe ampdb(-65), or lower)?
ringing - every analog amplification will ring slightly, so make that
lowpass a resonant lowpass, or maybe a few in parallel.

with higher fidelity gear, these effects are more subtle

On 3/23/12, richard duckworth  wrote:
> Tape compression, smoothing due to transformer balanced and/or discrete
> circuitry?
>
> Rich Duckworth
> Lecturer in Music Technology
> Department of Music
> House 5
> Trinity College
> Dublin 2
> Ireland
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>
> Tel 353 1 896 1500
>
>
> It's the most devastating moment in a young mans life, when he quite
> reasonably says to himself, "I shall never play The Dane!"
>
>
> ________________________________
>  From: Roger Kelly 
> To: csound 
> Sent: Friday, 23 March 2012, 16:30
> Subject: [Csnd] Tape vs DAW. Csound udo?
>
>
> Was wanting some "real" explanation as to why my mixes from my DAW sound
> better after bouncing to my 30 year old open reel tape recorder?
>
> Does anyone have a good tape udo for Csound?
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>

And there's also the effect of tape hysteresis...

Richard

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