Dorrough 400c meters

We just got a pair at work and they are huge... 

Just felt like sharing as maybe one day we can do a dorrough meter project.

I always felt like Dorrough stepped on my 1979 patent for peak/VU meters. (US04166245  Roberts). The main difference was that Dorrough used a curved display instead of straight line, and used some lawyerly synonyms for peak and average. I think they used "persistence" in their claims/description.

By the time they were selling their version, I (we) had lost control of my meter patent in a contentious bankruptcy auction. The guy who bought the patent had no idea what it was, and I wasn't about to tell him. So Dorrough went unchallenged.

Here is a picture of a meter I did for a friends console company several years ago (if it works). Console is still in production AFAIK.

JR

PS: I have kicked around the idea of making a generic meter chip (programmed micro) that could be combined with LED drivers to support multiple different meter configurations, kind of like a modern day  digital LM 3914/3915 soft programming pins could be grounded or floated to select, linear log step size, an A/D input could even program the step size (linear or ratio). The meter output could push out a serial data stream with 16 or more intervals, while the application could use as few of those as desired, ignoring the extra resolution.  I kept control of the source code in that console meter so It is free for me to use anyway I see fit, while this is not rocket science .

Always wanted one, need some kind of high rez meter

That is very cool work John!

Large, accurate metering is a wonderful thing in the mix room...  i too would be very interested in a project in this vein...   

Seeker said:

That is very cool work John!

Large, accurate metering is a wonderful thing in the mix room...  i too would be very interested in a project in this vein... 

Click to expand...

Back in the '80s I made a 100 segment vacuum fluorescent display prototype with 1 dB resolution over 100 dB dynamic range. That was tricky using old school analog and CMOS digital logic. I performed the linear to dB conversion by gating a timer on/off from the exponential decay of an RC (e^-t/RC). I won't vouch for meter accuracy down -100dB from full scale, but it was a pretty light show.

In practice I favor =dB steps so you can impute crest factor from the distance between peak and average. 16 LEDs with 3dB spacing makes a decent display (IMO), but my microprocessor approach could push out 24 steps or more, that you can ignore what you don't want (using serial com like SPI the extra LSB data that gets pushed out first just harmlessly spools off the end of the kept top MSB data). I like the serial in parallel out dedicated LED drivers.

JR

Sounds very very cool John! When can we see a black market offering? Or White?

BTW I can't see a pic in your first post...

Mike

Phrazemaster said:

Sounds very very cool John! When can we see a black market offering? Or White?

Click to expand...

I floated the idea before of offering a generic digital LM391x substitute, that could cover many bases using personality pins to select options, and was met with a compelling silence.

BTW I can't see a pic in your first post...

Mike

Click to expand...

I'll try to send over the meter image again... (above post image used a link to the image in another sub forum).

I fear this is would be too low volume to address seriously...

One could program an old school 8 bit 5V micro that could drive LEDs directly in a DIP package that DIY could easily incorporate into other designs. My preference would be to use a more modern 3.3V 16b micro with a LED latch-driver hanging off it... for audio, latching the LEDs should make less noise than direct drive perhaps needing multiplexing to address enough LEDs for a serious meter.

Not rocket science and perhaps a good project to learn embedded device programming with.

JR

I see that you used 16 LEDs in thin implementation.  Any reason it could not be increased to 20?
I love the design.  I ASSume it's constant current?
Thanks!
Best,
Bruno2000

bruno2000 said:

I see that you used 16 LEDs in thin implementation.  Any reason it could not be increased to 20?
I love the design.  I ASSume it's constant current?
Thanks!
Best,
Bruno2000

Click to expand...

The hardware used x8 HCT logic shift registers so 16 worked out neatly... 20 could have certainly been supported.  We used 3 dB steps so crest factor could easily be imputed from space between peak VU LEDs. (number of LEDs between PK and VU x 3dB = crest factor in dB. )

The 16 evenly spaced LEDs cover 48 dB of display range so +20 to -18 dBu or something like that. 4 more steps could extend that range 12 dB.  A  -30dB display floor might run into the sample and hold settling time issue (looks like crosstalk when high output level meter display is adjacent to a quiet meter display. ). This could be avoided with separate A/D for each channel of meter.  IIRC the processor I used could be partitioned as 1x 12b A/D or 4x8bit A/D, so perhaps that might resolve the sample and hold issue, but 8b x 6dB is only 48dB dynamic range, 7 bit after rectification so only  42dB... (note the display time constant math actually adds display resolution due to averaging over time, but I would be nervous about a display range larger than the A/D raw capture range.

A premium meter could easily afford to throw a cheap stereo codec  ($1 ?) at it, providing 2x 16b for more display range than any meter can practically use.

That older console meter used shift registers and current limiting resistors. I would use the more modern latch drivers that provide current source outputs.

JR