Stereo Threshold Compressor

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" height=90 BGCOLOR="#006699" TEXT="#080000" background="1fcfd5a3.jpg"> <div> <FONT SIZE="5" COLOR="#336666" FACE="Arial" ><B>BIT Company</B></FONT><B>     </B><B>|</B><B>     </B><A HREF="index.htm" TARGET="_top" TITLE="BIT Company"><U><B>home</B></U></A></div> <bR> <div> </div> </td> </tr><tr> <td valign="top" height=390 BGCOLOR="#FFFFCC" TEXT="#080000"> <div> <I>Stereo Threshold Compressor </I></div> <bR> <div> <IMG SRC="147fa540.jpg" border=0 width="250" height="84" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">  <IMG SRC="149fa520.jpg" border=0 width="250" height="82" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> Lord have mercy! There is probably no term an electronic engineer frets about more than "subjective", especially when applied to any audio system. It's even worse when said engineer used to be an audiophile. Messing with an audiophile's electrons can sure stir up a lot of dust!</div> <bR> <div> This gadget has made commercial satellite TV tolerable at last. Before you go much further, this device requires an audio breakout from your TV. This can be done quite a few ways and here's a partial list:</div> <bR> <div> 1) Stereo TV with a home entertainment audio loop (best)</div> <bR> <div> 2) Satellite receiver and separate stereo audio system (what I use)</div> <bR> <div> 3) Standard TV, mono or stereo with a headset jack. This is OK but requires a separate amplifier/speaker(s) as well. The headset audio output of the TV needs to be set at nominal line level.</div> <bR> <div> There has been nothing I've found more annoying than the wonders of digital television with the ability of the advertisers to zap up ad volume by quite often 400%. This was never possible with standard analog broadcast as the FCC was always rather tough on splattering transmissions.</div> <bR> <div> After about 8 months of "subjective" and objective evaluation, this design was deemed the best. There seems to be no real standard amongst the various satellite broadcasters, indeed, the measurements seem to show a rather callous disregard for their audience. One may be listening and enjoying "To Have and to Have Not", 6 dB below 0 Vu, then Mr. Billy suddenly appears at +10 Vu shrieking the wonders of "Orange Glop". Old folk like me don't need Ex-Lax, we got Mr. Billy! </div> <bR> <div> Back to the design, it proved fairly useless to use a fixed threshold to set the action of the dynamic compressor. There was just such a wide range of levels throughout the various stations that a dynamic threshold generator was the way to go. Since the aggravation was the sudden shock of the large volume excursions with certain ads, why not just limit them and not the program material? In order to follow the circuit description, you'll need to either print or open this<U> </U><FONT SIZE="3" COLOR="#0000FF" FACE="Comic Sans MS" ><A HREF="id47.htm" TARGET="_top" TITLE="Stereo Threshold Compressor schematic"><U><U>schematic</U></U></A></FONT> in another window.</div> <bR> <div> U2a and U2b form voltage followers with R4-R6 and R3-R5 working with CdS split cell VCR1 providing gain control from unity to approximately -20 dB as required by the loop. U2d sums the output signals of U2a and b and performs precision rectification which drives high speed thresholded integrator U2c. The output of U2c drives the LED in VCR1 illuminating the CdS split cell and reduces the input to U2a and b. D3 is simply a red LED in series with this drive, providing visual indication of function. As long as the average signal from the precision rectifier is lower than the reference on U2c pin 10, the entire circuit behaves as a "piece of wire" from the inputs to the outputs. </div> <bR> <div> U1a is another precision rectifier but it sums the input signals from the source. Instead of high speed integration, the output of this circuit is slowly averaged over approximately 2 minutes and drives voltage follower U1d. This level is the reference voltage for integrator U2c and is adjusted by R14. During normal program audio, a 50% setting of this control just touches peaks resulting in no gain reduction but will compress excursions within 30 ms when program audio suddenly becomes huckster audio. The averaged reference output of U1d will rise, therefore raising the reference voltage as long as the ad audio continues, hence, the output of the unit will slowly rise. When normal program audio resumes, the unit reverts to unity gain within 60 mS and the output of U1d slowly decays over the next 2 minutes, resetting the "yap-trap".</div> <bR> <div> U1c is a simple voltage follower which provides a Vcc/2 virtual ground for the unit. U1b was kinda "left over" and provides a high speed Vu output for those who might want to incorporate a meter or bar graph into the unit.</div> <bR> <div> Note: R14, the threshold level control, is not strictly required. For all of the program combinations I've listened to, R15 and R16 can be replaced with 62K resistors and R14 jumpered from U1d-14 to U2c-10. There are certain odd music channels where it is marginally useful to increase the threshold a bit and some where drastically reducing it makes the music more enjoyable!  </div> <div>  </div> <bR> <div> <A HREF="id48.htm" TARGET="_top" TITLE="Stereo Threshold Compressor layout"><U><IMG SRC="15b96640.png" border=0 width="150" height="100" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></U></A></div> <div> Circuit layout-click for larger image.</div> <bR> <bR> <div> Prototype photos:</div> <bR> <div> <IMG SRC="154fa180.jpg" border=0 width="250" height="280" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">  <IMG SRC="156a1180.jpg" border=0 width="417" height="280" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> <FONT SIZE="3" COLOR="#336666" FACE="Arial" ><B>Send me an email for a up to date component list at </B></FONT><U> </U><A HREF="mailto:bitco@alltel.net" TARGET="_top" TITLE="mailto:bitco@alltel.net"><U><B><U>bitco@alltel.net</U></B></U></A><B><U> </U></B></div> <div> <FONT SIZE="3" COLOR="#336666" FACE="Arial" ><B>and reference </B></FONT>Stereo Threshold Compressor <FONT SIZE="3" COLOR="#336666" FACE="Comic Sans MS" >in the subject line.</FONT></div> <bR> <div> <FONT SIZE="3" COLOR="#336666" FACE="Comic Sans MS" >If you wish to order engineering packages, kits, or assembled units, </FONT><A HREF="id49.htm" TARGET="_top" TITLE="Stereo Threshold Compressor pricing"><U><U>click here</U></U></A><U>.</U></div> <bR> <bR> <bR> <bR> <bR> <bR> </td> </tr></table></body>