FPGAARCADE minimig compatible board, comments?

[tonyyeb]

@ AJC

Lol! A moment of self realisation has just hit me!

[freqmax]

AJCopland, Ans she feels.. "hmm.. better find another boyfriend" .. ?  :-D

[mikej]

On a slightly different subject I'm just finishing the top mini break out card which has two front mounted joystick ports, composite and SVHS out and two audio RCA connectors.

At the moment the audio connectors are directly connected to the stereo jack out on the main board. I could add an output buffer / filter to these but I would have to either use a single ended op-amp and another output capacitor (not good) or generate a local -ve 5V supply. Any suggestions? worth the hassle / cost? The DAC data sheet is here :

http://www.wolfsonmicro.com/uploads/documents/en/WM8729.pdf

p.s. Does anybody have a 68EC020 processor I can buy / have? A broken A1200 main board would be a good donor if the processor is good.
/Mike

[AJCopland]

Not me but "Zetr0" over at the EAB probably does. I think he had a bunch of them for a project he / rkauer were doing for the A600. (if my memory serves me correctly)

Andy

[AJCopland]

freqmax wrote:
AJCopland, Ans she feels.. "hmm.. better find another boyfriend" .. ?  :-D

 :roflmao: probably, but I've learnt over time that when she starts to get a glazed expression it means I've been talking about the details of coding again :-D

Andy

[freqmax]

mikej, There are some ways to get that -5V:

(AB)use an MAX232 style capacitor chargepump to invert. Dunno how noisy it's output is.
You could use an Cuk converter or an Buck-Boost converter. And then a linear regulator following that for that low noise supply.

Another approach which I'm not sure will work is to put a small DC-blocking capacitor in series with the audio signal. And then arrange two (pos+neg) linear regulators to divide the +5V into +2,5V and -2,5V. Line-level audio signals are +/- 1V so it should be possible once you have the proper supply voltages.

In worst case.. just try

[mikej]

Thanks,
There are a few switching inverters which will do the job, but most are switched cap. Even if the switching frequency is quite high the supply will need some extensive filtering.

The DAC has quite a good build in filter, so is there any benefit in adding a line driver - especially if the op-amps power supply is not the best?
/Mike

[freqmax]

What is the reasoning behind adding another output buffer..?
If the switched cap need extensive filtering, go with an proper switched regulator and get the full juice.
Then use a heavy inductor and capacitor to filter. Don't forget radiated noise (shielding).

[mikej]

At the moment the DAC output is capacitive coupled directly to the stereo jack - which is the cheapest and smallest solution for the main card. The DAC has an output filter so it will work fine.

I have more space on the breakout board, and Wolfson talk about an optional low pass filter. It would also have low output impedance good for driving long cable runs.

Downsides are the cost (small) and the need to produce a -ve supply. It can be done with an inverting reg but they are a bit noisy, so without going overboard with the filtering the buffer may not improve the quality of the output, which is the aim for this optional card.

Thoughts?

/Mike

[freqmax]

Go with S/P-dif and get rid of the problem altogether ..

Don't forget that HDMI supports transmitting Audio on the same cable as Video.

Another option is to offer audio digital stream directly to an external connector and let and an additional circuit pcb handle it in any way the user see fit.

[mikej]

yes, the "big" expansion board will probably have USB, Ethernet and SP-dif. I just did the board symbol for the optical transmitter.

I can't use HDMI because of licensing issues. You can however connect my DVI out with a cable to a HDMI input on a TV, but no audio.
/Mike

[jkonstan]

Mikej,

DACs tend to have poor output drive capability; thus, adding buffers (after the series cap) to the DAC outputs is a good idea. The simpliest approach is using a single ended MC33204 (VCC and GND)with a "virtual ground (VCC/2)" as a buffer or as a non-inverting amplifier so that you will not need a -VEE rail.

http://www.onsemi.com/pub_link/Collateral/MC33201-D.PDF

[tonyyeb]

@Mike

Any news on a release date and final price?

[mikej]

jkonstan wrote:
Mikej,

DACs tend to have poor output drive capability; thus, adding buffers (after the series cap) to the DAC outputs is a good idea. The simpliest approach is using a single ended MC33204 (VCC and GND)with a "virtual ground (VCC/2)" as a buffer or as a non-inverting amplifier so that you will not need a -VEE rail.

Thanks, this was my thinking.
If I use a single ended amp then I will require an additional output capacitor? I was trying to avoid this as it degrades the low freq phase response further?

Any tips on how to modify the example WM8729 filter to run single ended? I've mailed Wolfson but no response as yet. I haven't done any analog design for a while, I avoid any components with less than 200 pins
/Mike

[freqmax]

As you already have the DVI connector everything needed for HDMI is present. Only an mechanical adapter is needed.

I haven't researched, but I think the restriction is to use the name "hdmi" in marketing. Or mentioning it in documentation.
I think you could say "DVI with sound".

So time for that secret "dvi sound patch"

As for analog solution, I think the most viable ones has been outlined by now. Personally an op-amp that manages with plain +V rail seems neat.

Have you tested if the output works for line-in on an standard amplifier?

[mikej]

The problem is the DVI driver chips I can use will not embed the audio, you can't buy them without a HDMI license. Nothing to stop you wiring the DVI up to a HDMI and using either a separate analog or the digital connection for the sound.

In the future I will look at direct driving the DVI from a Spartan6 with SerDes.

The standard stereo jack works fine, I have it wired up to my amplified PC speakers. Sounds rather nice

As soon as I have finished testing the ARM, memory controller and basic video I'll get some more boards made.

/Mike