From: lstowell@pyrnova.mis.pyramid.com (Lon Stowell)
Newsgroups: rec.audio
Subject: Re: Digital Amps -> next???
Date: 24 Nov 92 01:01:52 GMT
Reply-To: lstowell@pyrnova.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corp., Mountain View, CA

In article <1992Nov23.191010.24485@eng.cam.ac.uk> 91jcpk@eng.cam.ac.uk (J.C.P. Khoo) writes:
>
>Didn't Wadia introduce about half a year ago a true digital _power_ amp that
>used multibit switching to achieve power levels (c.15V and 2A)? I was intrigued
>by this amp because unlike all the others it was truly digital with even the
>volume contol implemented digitally.
>
    You are confusing the pre-amp portion with the power amp
    portion.

    Not that it is IMPOSSIBLE, but a true digital power amp would
    have only on and off states.  Whether you got your desired power
    levels thru multi-bit outputs or PWM would be up to you.  

    For example, you could have multiple power output sections (all
    hooked to the same load) with each section corresponding to a
    given volume/power level.  ALL of the sections would be switched
    on and off at the same time...   loud audio would switch on more
    of the power sections than soft audio.  But each section would
    be either fully on or fully off...    You could produce
    different frequencies by the rate at which you switch all the
    (active--determined by audio level) sections on and off...

    In any event, the filter components required to remove the
    digital artifacts from the output of the amp would be quite
    physically large--using TODAY's power output devices....and
    filter devices.   


>On a less esoteric scale, Meridian (from Boothroyd Stuart) have always been
>proponents of digital. To this end, they have produced the D600 and D6000
>digital loudspeakers. The D600 has an internal Philips bitstream converter
>whiuch feed a conventional poweramp and Xovers to the drive units.
>
   Those are NOT digital speakers, marketing-speak to the contrary.

   A digital speaker would have only on and off.  You could use a
   single driver and pwm it or multiple drivers in a multi-bit mode.


>The D6000 is another kettle of fish, it implements its Xover digitally with
>each unit having its own dedicated power amp optimised for its particular
>bandwidth and power requirements. To top it off, all the tone control and
>volume is implemented by DSP techniques.
i
   This is STILL all digital pre-amping, not true digital amp.

>
>Meridian also have a digital preamp the 601 (if memory serves) which has
>extensive DSP capabilities (including RIAA EQ,Dolby ProLogic, KEF KUBE EQ
>settings, and others). It accepts no analogue inputs. To connect a record
>player, you have to purchase their corresponding ADC (608?) to give it a
>digital datatream.
>
     
>All these products have made good impressions under subjective review and have
>even prompted favourable comments from die-hard analogue reviewers listening 
>to record players using the digital preamp and/or the digital loudspeakers.
>
    Given that NONE of the products you've mentioned are really
    "digital" power amps or speakers, so what?

>Philips have also introduced their versio of the digital loudspeaker, but
>details of this escape me at this moment.
>

   NO, they haven't.  The only folks I know who've actually
   implemented a TRUE digital speaker are the folks at Bell Labs
   [or whatever their name is these days...,]

   Remember what DIGITAL means.  A digital speaker would not have
   infinite variable position (like a current speaker)....it would
   either have it's driver fully in one position or the other.  

   Different frequencies can be generated by pulsing the speaker 
   widget fully forward or fully backwards at different frequencies.

   How would you produce different volume levels?

   One way is the way the Bell labs folks did, by having different
   SIZED drivers...all of which move at the same frequency.  For
   soft sounds, the small drivers move, for louder sounds, the
   larger drivers begin moving.  But ALL of the drivers always move
   exactly the same amount for each driver.   

   This of course tends to create a lot of clicking and clacking at
   best.  A crude mechanical filter can remove this, leaving the
   audio.  

   The amazing thing is that the thing worked at all.....