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From: djh@dragon.metaphor.com (Dallas J. Hodgson)
Newsgroups: comp.sys.ibm.pc,rec.games.programmer
Subject: Re: Quality of PC sampled sound
Keywords: samples, sound, IBM-PC, Amiga
Message-ID: <1207@metaphor.Metaphor.COM>
Date: 11 Jun 90 19:11:50 GMT
References: <IaPQ7gi00WB_A9tKcj@andrew.cmu.edu> <1990Jun7.173911.424@cbnews.att.com>
Sender: news@metaphor.Metaphor.COM
Reply-To: djh@dragon.metaphor.com (Dallas J. Hodgson)
Distribution: na
Organization: Metaphor Computer Systems, Mountain View, CA
Lines: 61


I recently wrote a few support routines for reproducing sampled sound on the
PC. I used a Mimetics digitizer to capture the samples (8-bit, around 14KHz)
into an Amiga, and compressed the bits like so:

 for each byte that exceeds the 0 crossing, output a "1" bit
   else output a "0" bit.

This stream of bits would get bit-packed into a byte stream, translated into
"C" source, (a large static array) and subsequently shifted out into the
speaker port of a PS/2. My player looks like this:

PlaySample(buffer,size,speed)
unsigned char *buffer;
unsigned short size;
short speed;
{
  char x; /* must be unsigned! */
  unsigned char tmp;
  unsigned short bitctr,dly;
 
  /* Samples are stored internally as bit-packed values; each PCM value
     appears in Bit 7 down to bit 0, the starting again in the next byte.
     Packing the bytes this way gives us an easy way to check the each
     bit (via the sign) before it gets shifted out. */
 
  while (size--) {
    x = *buffer++;
    bitctr=0;
    do {
      tmp=inp(0x61);
 
      if (x < 0) { /* bit 7 set? */
        if (!(tmp & 2)) outp(0x61,tmp | 2);
      }
      else  {
        if (tmp & 2) outp(0x61,tmp & 0xFD);
      }
 
      x<<=1; bitctr++;
      for (dly=0;dly<speed;dly++);
    } while (bitctr<=8);
  }
}

My question is this : Why does it sound so poor? Compared to some other PC
sampled sound, the distortion is really bad; a very audible aliasing-screech
overtone is present on top of the sample regardless of anything I try.

Regarding the sample itself: I used a 2-second sample of speech, taken from
a comedy CD and run through a compressor. Frequency content is well below
7KHz. For reference, I remember a PD program called "helpme.com" that had
excellent clarity. Thanx in advance,
+----------------------------------------------------------------------------+
| Dallas J. Hodgson               |     "This here's the wattle,             |
| Metaphor Computer Systems       |      It's the emblem of our land.        |
| Mountain View, Ca.              |      You can put it in a bottle,         |
| USENET : djh@metaphor.com       |      You can hold it in your hand."      |
+============================================================================+
| "The views I express are my own, and not necessarily those of my employer" |
+----------------------------------------------------------------------------+