The Expressionist, a review

[BJ]

Cary Roberts wrote:
> 
> >Which one , a 68302,68332,68HC12 etc ???
> 
> 68000
> 
> >Thats not all of the truth, it depends on what the box have to do,
> >LFO's,HZ/volt etc and it depends on how well written the code is.
> 
> I think the 68000 was a good choice.  It's no more costly than a 6811
> or Z80 or 8051.

It may and may not be a good choice because you would need  
separate UART IC, a buss ACK/ready cirquit, and some extras additionals.

And you canot compare a Z80 to a 6811 to a 68K,its completely
in different ballparks in the price range for example.

The 6811 is a MCU the Z80 and 68k is CPU's.
The Z80 lacks many instructions and adress modes who makes it
slow compared to a 6811,8051, 68k.

The 6811 is wery efficent for controll things.
 
> >Ouch!! costly!!
> >But there is up to the PCB designer to do a good power layout if
> >mixed technology are used, usally a +/-1 mV of power noise
> >are not causing any problems ! :)
> 
> The power supply section has separate voltage regulators and filter
> caps.  That's good practice, the orginal poster made it sound like
> they were totally separate, they are not.

It doesn't matter, if the power and ground layout of the PCB is laid out
in the wrong way so the return currents (as discussed erlier on this
list)
takes the wrong path to the summing node.
 
> >> The D/A convertor on the Expressionist is 16 bit.
> >Which one??
> 
> I can't remember off the top of my head, but it was either a Sony or
> a Burr Brown.

OK!
 
> >Many boxes uses 8 bitters infact,and then you have to use multiturn
> >pots to set the scaling of the CV. many boxes uses 12bitters (as mine
> >does).
> >Most 12 bitters uses software scaling of the CV's, depending on the
> >range
> >the DAC has to work at, 0 to + 8volt as a example.
> >And as you say some uses 16 bitters.
> 
> It takes at least six bits to do a 64 note 1v/oct scale.  8 bit only
> leaves .25 semitone accuracy.  That is pathetic at best. 

No i dissagree, it depends on the linearity of the DAC too, both PAIA
and Roland MPU401 and others uses 8 bit with exelent results.
 
> 12 bit ups
> that to 1/64 semitone accuracy, which while good, can still cause beats
> between identically tuned voices.  With a 16 bit D/A Tony Karavidas
> (the designer) was able to handle note, pitchbend, modulation, and
> scale adjust with just one D/A conversion. 

Still, it depends on the voltage range the converter has to cope with.
My Midi CV converter do that to in just one D/A mux update conversion.
It depends how well the code is written too.

With a 16 bitter for a 10 volt range we are down to 0.152mV
a DA stept, this is far below the offsett voltage on a high quality
OP amp. 

I would arugmenting that its extremely difficult to get this kinda
low AC noise voltage in a odinary power supply as a 16 bit DA are
kapable of!

> Older boxes (and some  current  ones too) use separate D/A cycles
> for both note and modulation.  That
> can induce error in the summing amp and allows for more droop error
> due to the extra demux/S&H channel.

??? I dont understand what you are talking about here??
Usually you do all the calculations in the CPU and then output a
"complete" CV voltage to the mux cannel who has to be uppdated.
This CV voltage contains all MIDI information selected to this
specific CV channel, then its just up to the CPU to keep
the CV channel refreshed so it would not lose the voltage and start
drifting. 

I dont see the picture where you have a summing amp?
A 68k 8Mhz CPU has the power to uppdate more then 128 CV channels
and do some heavy syntheis callculations at the same time without any
Mux doppouts.
 
> >In a FPGA?????
> >That is strange , are you sure on this , perhaps you ment "Flash" or
> >EEPROM!!
> 
> There's a PIC in there, I don't remember an FPGA.  I could be wrong.
> It definitely stores the operating system and parameters in
> FLASH/EEPROM.

What does the PIC doing in there?
 
> >Thanks for the review!
> >I to think Encores CV converter is among the best, sad its "wery"
> >expensive here!
> 
> Not only is Encore's CV box an exceptional product, the MIDI retrofits
> are outstanding too.  They replace the original processor and ROM with
> a new board, (relocated) processor, ROM, RAM, and UART.  This
> is a FAR BETTER route than Kenton takes.  I highly recommend the kits.

Kenton uses loads of electronics to do the things to be done(at least in
the older models)

BJ