Prelude Zorro 2 Amiga sound card - part 2 MPEGit FIX

[@Tjuwantjutju]

I love to tinker with old machines They have a charm that never fades I fix them up with care and skill And bring them back to life again Amiga, Atari, Commodore They are my friends from long ago They make me happy when I play The games and programs they can run I don't need the latest tech To have some fun and feel fulfilled I have my old computers here They are my passion and my joy

[@TLang-el6sk]

According to the datasheet there are two registers with PLL offset values for 44.1 and 48 kHz. These are loaded with the values for the given 14.750MHz oscillator. They could be modified for other oscillator frequencies. As long as the drivers leave these registers alone you could write a program setting the correct values for the internal processing and the output should be correct.

[@CRG]

A neat idea but I wouldn't know where to start. I think for me it will be simpler to try and get the .724mhz oscillator.

[@pvc988]

Try replacing caps in microphone input op-amp feedback (C41 and C42 in the schematic I have; 1.2nF in value) with 680pF. This should improve high frequency response.

[@CRG]

Will do, thanks for the tip

[@weepingscorpion8739]

Too bad about the MPEGit card but that is still a great soundcard overall. And also thanks from us viewers to Mrs. CRG. Seeing that microscope more in the future will be great.

[@CRG]

Yeah the microscope will be very useful I'm sure. Still need to try it's direct record feature but I think it'll work great for future projects.

[@42markk42]

Hi Glen, the extension ribbon cable problem surfaces elsewhere too, as I found out when trying to extend my CF card from my Terriblefire 1230 out to the back of the case, just wouldn't play ball at all. Had to use a really short cable which didn't reach the back. All academic now though, as I just fitted a PiStorm32 lite with pi4 , and have a SD/USB/HDMI plate there now.

[@SudosFTW]

Yeah it seems like the closest you can get it 14.71 or 14.74MHz, in 3.3v form and SMD for expensive money. I looked to see if one could get a 29.45MHz oscillator as it wouldn't be too hard to make a frequency divider, but alas, same thing. Your best bet is going to be a programmable PLL of some flavor that can output the desired frequency if you want playback to be spot on. The other thing to check out is if the audio track you're playing back is VBR, some MP3 decoder chips don't like VBR too much for some strange reason and will run those files slower.

[@CRG]

We've found a programmable oscillator from digikey that should do the job. It's a little expensive but I'm keen to know if it'll fix the issue.

[@dh2032]

​@@CRG could you not use a stripped down raspberry pie Pico, cranked up as fast as it will go, 3.3volts be in the range of just pin connection? on a Pico?

[@AnnatarTheMaia]

The prelude microphone input is really muffled, almost like there is impedance somewhere.

[@TLang-el6sk]

Another remark: Perhaps you can use the SIT8008BI-23-33E-14.725000, but it's SMD.

[@CRG]

An smd part would be fine, I'm sure I could attach it with wires. But we've found a programmable oscillator that we can get from digikey. Think we might be going that route.

[@DAVIDGREGORYKERR]

What about a synthetic crystal using a VCO with a MOTOROLA Synthesizer chip and a 10.240Mhz crystal should allow you to generate the required clock signal.

[@cullmaster7361]

It’s interesting. Have just seen posted that AHI V4x should be used and NOT AHI V6x for these sound cards 🤔

[@CRG]

As I understand, that's correct for the original driver but for the new driver ahi6 is preferred.

[@jkdsteve]

It would mean unsoldering the MPEGit but I bet you could get away with using a right angled pin header to mount the MPEGit, so long as the Prelude is not flush against the case already.

[@Voyager_2]

What is the sample rate you record the mic in signal at? Because it sounds like 11Khz

[@LeftoverBeefcake]

It almost sounds like it's being recorded by the Paula sound chip and not the Prelude card. I wonder if there is a software setting that could be twiddled... maybe a pass-thru is turned on...?

[@Voyager_2]

Paula can not record.@@LeftoverBeefcake

[@kaitlyn__L]

Didn’t the Amiga use ~8.5kHz? But I’d hope an expanded system like this wasn’t limited to that.

[@Voyager_2]

The prelude can do up to 64 kHz playback and record amiga resource says. But I know from memory that it can do 44kHz recording with ease and without any cpu overhead. @@kaitlyn__L

[@CRG]

There is no setting in that ahi recording software to set the sample rate, it's just recording a raw wave file. I have tried some other more professional software and it doesn't sound any better. I'm going to try tweaking the caps on the mic input (as suggested in another comment) to see if I can improve it.

[@lemdixon01]

Better to press down bent pins with tweezers rested on the table.

[@CRG]

Good idea, I'll keep that in mind for next time 👍

[@lemdixon01]

The crystal should be in circuit with a couple of small surface mount capacitors. If you change the value of these caps it might alter frequency and speed of the music but it might not as the frequency might depend upon the fixed frequency of the crystal and if you change the caps to different values it might just not work. If you do end up changing the crystal to one with a different frequency, you might have to change also any caps in parallel with it

[@pvc988]

It has complete oscillator module and not just bare crystal. I doubt it's tunable.

[@TLang-el6sk]

That four pin oscillator module is not tunable, the only cap useful/required is the typical 100nF bypass capacitor for the supply pins...

[@CRG]

As the others have said this oscillator can't be tuned but we've found a programmable part that we can order to whatever frequency we want so hoping that'll fix the issue.

[@mikedefoy]

Ribbon cables caused me endless problems. A needle nose pliers across all side of legs works well for straightening purposes.

[@CRG]

Bit of a shame I can't get this mounted as I wanted to show it off but these things happen, it was never meant to be mounted like that. At least its working now. I'd have to get myself some better pliers to equally grip the pins on something so small but yeah, with a decent set I'm sure that'd work.