Composite Video from the Nintendo Gameboy

My latest project is to get some sort of video output from the nintendo gameboy. The signals are all present, but are not in a form that can be connected to any television or monitor. My first thought is to convert the signals into a single analog signal called composite video.

Composite video is named as such because it contains color data (chroma), brightness (luma) and synchronization on wire.

1. Chroma is not important in this case becuase the gameboy is monochrome. All of the color differences will be made with Luma.

2. Luma: As above, the Luma will be used to create our four different colors. The TV looks for an analog signal here. The gameboy has two data pins which are used for this purpose, so connected through a DAC of some sort, we can create four different levels rather than just 0v and 5v.

3. Sync
 Csync or composite sync is an XOR of horizontal and vertical sync. These two signals tell the television to move down one line or to move back to the top of the screen. I am not quite sure how progressive or interlaced are determined by these just yet.

Next, these have to be connected together over one wire. As mentioned, the luma is an analog value but what I forgot to mention was that it is a positive signal whereas sync is negative. From what I have read, composite must be AC coupled because the TV input is DC coupled. That being said, our sync can actually be positive as well, as long as the luma is offset by DC value. Once the signal goes into the TV, the DC coupling circuit will block the average DC offset which is our black level. Black level will become 0 volts inside the Tv so sync will become 0v to -0.4v and Luma will be between 0v and +0.7v.

I am not at liberty to share my schematic at this point but it doesn't' work that well anyhow. :P
As a first attempt however, the result is quite promising.

You will notice that the image scrolls in several directions and that it is repeated three times over from left to right. I believe this is a problem with synchronization and possibly the LCD itself.

Updates to follow shortly. 

Miracle Piano Final Thougts

I finally had time to probe a few pins inside the Miracle Piano.

AS0012 - Pin 21 (PWM)
It turns out this pin controls the master volume and NOT the envelope as I had originally assumed.
Turning on the keyboard, this pin defaults to about 50%, but as you can see, it is not exactly 50%. How odd.
When the volume is reduced, the pulse width is widened and when the volume is increased, the pulse width is reduced. That is because the integrator, U4A is inverting the output.

Each photo has the scope set to 10 microseconds per division:

Default Volume:



Highest Volume:



Lowest volume:



U4 - Pin 1
This pin goes to the LM13700 to control the master volume, or gain of the sound. It appears as a DC voltage and is remarkably solid. There is no visible ripple as would be expected from an integrator.
No photo is needed of this signal.

J004 - Pins 21-24 (OUT1-OUT4)
The signals from these pins are too small to view on my scope, so I chose the amplified versions from U10. Pins 1, 8, 7 and 14 respectively.

OUT1 is not active for all sound samples, apparently. When Harpsichord and Synthesizer are selected, nothing comes out at all.
When the other four sounds are selected however, the waveform is perfectly clear.

OUT2 is the opposite. Only when Harpsichord and Synth are selected, do we see any activity on this pin.

OUT3 mimics OUT1 exactly, or at least as far as I can tell.

OUT4 on the other hand does not seem to react to any button or key. It simply outputs a DC voltage which measures 5.03v at pin 14 of U10.

I can speculate as to why there are three OUT pins. While probing both Pins 8 and 14 of U4 which has the mixed and filtered outputs split to both left and right output terminals, the waveforms are identical for Harpsichord and Synthesizer but are not identical for the other four sounds. It would seem that Harpsichord and Synthesizer were recorded and digitized in mono while the other four sounds were recorded and digitized in stereo.

I am a little saddened that the envelope is not broken out to any specific pin or sub-circuit.

Some other Notes

J004 - Pin 40 (Bus Strobe)
This pin reacts differently for different sounds, however, when the key is pressed with a sound that has an attack and release; the pin will strobe for the full length of the sound and go to 0 when the note is finished.
For a sound that ends only when the key is released, the pin strobes at key press for a moment, then goes to 0 and strobes once again when the note is released.

I could use this pin for creating a gate and trigger, but it would require that I charge a capacitor then feed it through a comparator and set up some logic to keep the signal high when the key is pressed and low when the key is released. This wouldn't require too much effort, but the function would change from sample to sample since the envelopes are all different. I don't thin the modification is worth it in this case.

"Piano" sample is very hard to photograph since the amplitude changes so rapidly in software. It looks very similar to "Organ, but is more rounded whereas the Organ is more triangular.


"Organ:"



This photo shows a sample that is identical on both sound channels. Trust me, they are identical but the channels are not being displayed the same way.


These two photos show sounds that are not identical from OUT1 and OUT2.



PS: Yes, the reflection in my oscope is naked. Deal with it.

min64 - The bare minimum rom cart for commodore 64 - part 2

The boards arrived but I have yet to test them out.

min64 - The bare minimum rom cart for commodore 64

The idea was to design a cart that would fit entirely in the commodore without sticking out. Surprisingly, it could even be made small enough to fit a shell, if I make one in the future.

The board has has one ROM and thats it.








The board is a meer 1.05 inches deep and has notches for easy removal or mounting (granted a shell is designed). Measuring my own commodore, a board of 1.1875 inches would be flush with the chassis. Also has optional reset switch as suggested by catskull.

My main concern was supporting as many proms, eproms and eeproms as I could since too many are no longer made, but easy to find.

Those supported include:

28F010, 28F020, 28F040,
29F010, 29F020, 29F040,
49F010, 49F020, 49F040
2764, 27128, 27256, 27512
27c64, 27c128, 27c256, 27c512

As well as SF's, GL's, EE's, etc.

Also working on support for 2704, 2708, 2716, and 2732. These will fit, but would require a few rerouted pins.

In any case, I can't wait for these to arrive from OSHpark.

CCAM - Crystal Clear Audio Mod for Sega Genesis/MegaDrive by Tiido

I took an afternoon to look at Tiido's circuit to clean up and amplify certain channels on the poorer created versions of the Sega Genesis and megadrive. This circuit separates all of the audio channels, amplifies them individually, mixes, amplifies again, offsets the DC output and applies a low pass filter. This is based on the sound filter inside of the original "HD Graphics" models of the sega genesis which arguably has the best sounding output.

I have taken the liberty of redesigning the circuit with all higher quality 1% tolerant, through-hole components, a low power op amp and easy to solder tie points for installation.

Fun with the Sega Genesis - Ultimate Portable Game player

Found a crazy deal on fleabay for one of these and had to have it. I had heard stories of how badly they are made, and how miserable the audio is.
All very true, but I think it can be improved a little.

The video is great, the size a too small, the sound from the speaker is crap but the sound from the headphone could be worse.
I immediately took it apart after opening the box. I was THRILLED to see only one epoxy-chip and several SOP's! :D

I want to believe that the audio issue is because of the amp/filter they have used. Through headphones, it sounds ok, but also a little clipped and shallow. This could also be because of the power supply, which is very weak. The battery is 3.7v and droops when the PSG is loud.




Its like a portable MP3 player considering all the titles that have sound tests. :p
You know, except horrible sounding.

Do not stay at the Guest Quarters Hotels - Dumping The Miracle Piano Teaching System

I had my Willem programmer pulled out for some Sega Genesis programming stuff and decided to finally dump the eprom inside of the Miracle Piano teaching System Keyboard. If you recall from a previous post of mine, it contained a 27c256 OTP eprom. It also contained many proprietary ICs of which I may never figure out. We may never know what microcontroller they used or what language the machine code is suppose to be written in. z80, 6502, etc.

In any case; with many vintage computers and other devices containing ROM, the authors always find the space to sign and date their work and in rare cases they may even decide to add a little easter egg. Although I cannot locate a date, here is some clear text I found in the ROM image:

"This Eprom contains code created by Mike Collins. Anne Graham and Ray Livingston. Do not stay at the Guest Quarters Hotels. Keep that Coke classic and that Piping hot coffee coming."



I got a real kick out of reading that. For anyone who knows what to do with it, here is the binary file:
http://www.mediafire.com/download/eo3zujca7jvw44f/MPTS_ROM.zip