Snowball Sunday

As it's almost December, let's dig out the awesome Snowball Sunday for the Commodore 64 by Ash & Dave, released 1988.

I first played this on a Commodore Disk User cover disk, Volume 2 Number 2 Jan/Feb 1989.  It's on Side B of the physical disk or cdu202b.d64 if you've pulled the image from the CDU archive on the web.

Also check out the game Microdot from side A if you get chance, especially the level editor.

Snowball Sunday (Commodore 64) 1988

Commodore Disk User Vol 2 Num 2 1989

I was specifically looking for the Lou Gray SID file used on the title screen (space.sid).

From HVSC the two SID tracks are:
/MUSICIANS/G/Gray_Lou/Space.sid
/MUSICIANS/B/Beben_Wally/Winter_Olympiad_88_preview.sid

Commodore 64 High Resolution Audio Capture

In my last post I captured the audio output from an Amiga A1200 using a M-Audio ADC.  For fun, here's a few lossless recordings I made of my favourite Commodore 64 tunes.

These SID files were played on a real Commodore 64c fitted with a MOS 8580 SID chip, played from Sidplay64.  Audio  is captured from the AV DIN jack with a M-Audio M-Track MkII and recorded using Audacity on Linux at 48kHz 24bit PCM.

Google Drive Link for C64 FLAC files

Files:
C64_Cybernoid_II.maudio.flac               C64_Ocean_Loader_2.maudio.flac
C64_Deflektor.maudio.flac                    C64_Ocean_Loader_3.maudio.flac
C64_Equinox.maudio.flac                      C64_Out_Run_tune1.maudio.flac
C64_Finders_Keepers.maudio.flac         C64_Out_Run_tune2.maudio.flac
C64_Flight_Path_747.maudio.flac         C64_Park_Patrol.maudio.flac
C64_Mask_II.maudio.flac                      C64_Rebounder.maudio.flac
C64_Monty_on_the_Run_High_Score.maudio.flac  C64_Thing_on_a_Spring.maudio.flac
C64_Monty_on_the_Run_Main.maudio.flac   C64_Wizardry.maudio.flac

Sidplay2 on C=64

If you enjoyed these files, there's a project to record the whole of the HVSID collection from a real C64 called the Authentic SID Collection: http://www.6581-8580.com/
 

M-Audio M-Track MkII

 
Enjoy!

Amiga A1200 High Resolution Audio Capture

I'm working on a customer's M-Audio M-Track MkII and couldn't resist the opportunity to test it by using it to capture the audio output from the Amiga A1200.  This is the same Amiga 1200 with the Rev 1A board which I've blogged about previously.

Amiga A1200 (ignore the MIDI cable, that's for a later post)

The M-Track MkII is a 2 channel analogue to digital converter and I have it connected to a Linux PC running Fedora 26 which immediately recognised the device and it showed up as as both a stereo input and output device in Pulseaudio.  I used Audacity to record and edit the audio at 48kHz stereo, exporting as 24 bit FLAC.

M-Audio M-Track MkII

The M-Audio M-Track is a pretty awesome bit of kit and worked well under Linux.  I was even able to play audio to the device and mix between Direct (analogue) Input and USB using the front panel mixer control.  Very nice indeed.

The Amiga audio captured directly from the rear panel L+R phono sockets also sounds amazing.  Unfortunately I don't have another A1200 to compare with so I don't know how this Rev 1A computer compares with later hardware revisions.

You can download a selection of audio files in FLAC format here which are obviously quite big being lossless:

Google Drive - Amiga A1200 Audio Files

The files available are:

• A1200_mod.chippermodphosis_maudio.flac
• A1200_mod.fresh_house_maudio.flac
• A1200_mod.human_invasion_maudio.flac
• A1200_mod.milk_punch_maudio.flac

These are just a few of my current favourites which I've been using for testing and repair of this particular A1200.  Only Amiga makes it possible!

Amiga A1200 Real-time Clock Module (RTC)

The Amiga 1200 doesn't have a battery backed real-time clock fitted by default.  It's shown in the schematics as an optional item but was never installed in any of the board revisions to my knowledge.

Some trapdoor expansions have a battery backed RTC fitted but I don't have one as yet.  I opted for a small battery backed RTC that's installed on the clock header next to the Kickstart ROMs.  I went for the Individual Computers RTC as I particularly like the products Jens produces.

Here is the board...

A1200 battery backed real-time clock module

And here it is installed in an Amiga 1200 with a Rev 1A board.  This 1A board has the full width header pins installed so the board is installed using the right most pins.

A1200 with battery backed real-time clock fitted

Firing up the Amiga I was able to set the time & date and save it to the RTC with "setclock save".  AmigaOS was able to see the fitted RTC and read back the correct time/date with "setclock load".

However, the following day when I started up the Amiga, the clock had jumped back to an earlier date as shown below.  The 3.1 ROM behaviour appears to be loading the clock from the battery backed RTC module, but if it returns a default value because the hardware has reset, the OS instead uses the DATESTAMP of the boot disk instead, which in my case was the installation time of the hard disk drive device DH0.

Amiga setclock with failed real-time clock battery

As the 3V battery was brand new, I initially suspected the RTC had a failed oscillator and wasn't incrementing when the unit was powered off.  However, this is easy to test by writing the correct date/time to the RTC (setclock save) and then periodically reading it back (setclock load) and checking the OS clock with "date".  In this case the RTC was working fine but was losing it's settings when the Amiga was powered down which indicates a problem with the battery backup.

Removing the RTC board I measured good battery voltage (3V) but wasn't reading any backup voltage on the clock IC itself.  On closer inspection it appears the battery holder possibly wasn't soldered to the board properly as shown...

Re-solder the tabs on the battery holder

After the repair I was able to measure 2.4V on the clock IC (there's 0.6V drop across D1).  The Amiga now seems to remember its clock properly on a cold boot as shown.

Amiga setclock with correctly working RTC

Amiga 1200 Rev 1A Distorted Audio Output - Bonus Material

Here is some additional information related to the Amiga 1200 audio fix described in my previous posts

This is the PCB audio section with the completed modifications.  Note that R325 and R335 are on the bottom side of the board, the other modified components are on the top.

A1200 audio opamp and filter section (top)

A1200 audio opamp and filter section (bottom)

The following oscilloscope images show the audio output at the phono jacks on the rear of the unit after the final Rev 2 board modifications.  The Amiga is playing a 1 kHz test tone at maximum volume.

The first image is with the low pass filter disabled (power LED dim).  The second image has the low pass filter enabled (power LED bright).

Low pass filter disabled

Low pass filter enabled

The Amiga schematics for each board revision are available online.  For reference here are the Rev 2 schematics for the audio section (Paula) with the changes from the Rev 1A board highlighted.

A1200 Rev 2 Schematics for audio section

Amiga 1200 Rev 1A Distorted Audio Output - Part 4 (Final)

This is the final instalment of my quest to get nice sounding audio out of an Amiga A1200 Rev 1A.

Here is the same mod file captured at 0dB with the following modifications:

R331 changed to 680R
R321 changed to 680R
C331 changed to 6800pF
C321 changed to 6800pF
R335 changed to 360R
R325 changed to 360R
R345 changed to 680R

(R335 and R325 are on the bottom side of the board)

These modifications are the same as Commodore implemented between the Rev 1 and Rev 2 boards according to the schematics I have.

For comparison with previous posts, here's the 1kHz sound test with the LED filter disabled (jaggies) and enabled (smooth)...

The audio output sounds about perfect to me and I'm more than satisfied with the result.  It does appear the changes Commodore specified for the Rev 2 board onwards were well engineered.

The output level is still a little higher than the A600 (Rev 1.5 board) and could perhaps be lowered just slightly (say changing R331 and R321 to ~600R).  Also, with the optional (LED) filter switched off there's noticeably more aliasing present than on the A600.  However, I think these slight differences give the A1200 its own characteristic sound verses the other models and I rather like it.

In theory this machine should sound the same as a stock Rev 2 board but I don't have one to compare with.

That's the end of this mod.  Thanks for reading!

Amiga 1200 Rev 1A Distorted Audio Output - Part 3

Here's the same mod file being captured at 0dB after the following motherboard modifications:

R331 changed to 340R (2 x 680R in parallel)
R321 changed to 340R (2 x 680R in parallel)
C331 changed to 6800pF
C321 changed to 6800pF

Gain is significantly reduced although still quite a bit higher than the A600 (Rev 1.5 board). I'd say the playback is 99% correct and I could easily use it like this but the gain is still just a tiny bit too high for my taste.

Another notable difference between the A600 and A1200 is the aliasing present when playing the 1kHz test tone with the low pass filter disabled (dim power LED).

Here's the full volume 1kHz test tone captured at 0dB.  Note the level is now much reduced and there's visible aliasing on the waveform.

Here is the same test with the low pass filter switched on (bright power LED).  Note the nice clean waveform.

I might have another go at reducing the gain a little more and trying to clean up the default filtering to remove the aliasing.