unused compandor pins ne571

[kingswayguitar]

hi
been searching around the net and cant find where this is answered explicitly. new to companders and experimenting with the ne571. i started with 1/2 of the chip ( pins 1 to 8 ). do the unused pins on the other half need connections or can i leave then open? following the input compressor (ic2) of the dm-2. top of schematic here http://www.hobby-hour.com/electronics/s/dm2-delay.php

by the way, what is the other half of the chip doing in that circuit?
thanks

[Mark Hammer]

Companding is a "dual-ended" process,so "the other half" of the 571 in that circuit is engaged in the complementary action.  One half of the chip compresses the signal, such that the quietest passages are well above the noise floor, but do not exceed the headroom of the BBD.  The other half of the chip expands the dynamics of the post-BBD signal so that original dynamics are restored.

[kingswayguitar]

thanks. so you're saying that just using half the chip in a compressor topology is flawed? like i said, i know little of companders.
peace

[Mark Hammer]

Not necessarily flawed.  For instance, the Boss Dimension C takes a mono input and provides a stereo output.  Because of all the stuff in there, and the robust opportunities for heterodyning of the dual clocks, it uses companding.  But because it's mono-in/stereo-out, it uses two compander chips, and leaves half of one sitting idle.  Compression on the input, and separate (but equivalent) expanding on the dual outputs.

I'm just mentioning that because there can be sensible reasons for using only half of a compander chip.  Depends what you're trying to achieve.

[kingswayguitar]

thanks again. i'm just experimenting. slowly getting into delays which use companders. the compressor on its own sounds crappy but i see how it would be useful. long ago i had a sustainer that used one. somehow it maintained the harmonics though.

i started with the boss topology and will try the datasheet example next. afterwards i'll fiddle with some component values then put it in an actual circuit, maybe a pt2399 or bbd.

-ben