8-BIT DIGITAL TO ANALOG CONVERTER
Micro Technology Unlimited — $50
Review by Arthur Hunkins
Micro Technology Unlimited has produced an excellent DAC board for advanced home music applications. It contains an on-board filter and audio amplifier, and extends its connectors (user and cassette ports) to fingers on the opposite side, an important advantage for new PET's. It additionally provides a CB2 input to the audio amplifier section, bypassing the filter, and facilitating the board's use in simple, traditional musical applications.
Technical specifications and overall design are superior. The unit comes with a good manual that includes schematic, board layout, parts list, principles of operation, troubleshooting guide, and a modest amount of installation and test information. One problem: only a two-line program is given to verify correct operation of all sections, a program that, if successful, produces the sound of a "misfiring race car!" No other user software is included, only the suggestion that you purchase the K-1002-2 Advanced Music Software package for $20. (This advanced software package, which will be the subject of a January review, is entirely in machine language, and like many sophisticated programs, will not yet work on the new PET's. It does demonstrate, the capability of producing four-part harmony, each part with a unique tone color.)
For any musical application short of a stand-alone composition, use of the HUH Petunia software is recommended. It runs without modification on MTU's D/A converter. The Petunia software is a straightforward PET adaptation of one of Hal Chamberlin's Sept. 1977 Byte magazine programs. (This important article of Chamberlin's, "A Sampling of Techniques for Computer Performance of Music," reprinted in the Byte book of Computer Music, is the source for 6502 microprocessor musical applications, including HUH software and hardware, and the various products MTU is marketing for the Kim and PET. Of course, Hal Chamberlin is Mr. Music at Micro Technology.)
The Petunia software (also in machine language though loaded by BASIC) can be easily modified to fit in the upper .5K of user memory, rather than 1K. Unfortunately, even this otherwise highly useful sound generating routine must be substantially modified to run on new PET's.
The MTU DAC is powered by the PET's own + 5 volts at the cassette port, a significant design feat for both filter and amplifier. It consumes little power (quiescent: less than 50ma; worst case drain: 300ma). The filter cuts off sharply at 3.5kHz, and is of a six-pole, .5db Chebyshev design. Power output is 300mw into an 8 ohm load (4-16 ohm permitted). The output features on-board trimpot and RCA phono jack. I use a modified ($6) Realistic Junction Box with two set of headphones, switchable to a small speaker. The manufacturer suggests hookup to an external amplifier for use requiring greater volume. One additional plus: jumper connections are readily accessible for obtaining at the output jack either the raw DAC, or unamplified filter signal.
Minor hardware disadvantages are as follows: 1) the connector finger extensions are not keyed (a hacksaw cures this problem fast), and 2) the cassette deck fingers are brought out on the lower side of the board. Since external cassettes use the upper connections, this is a notable inconvenience for those intending to use the board (eventually!) with the new PET's. However, a modest amount of rewiring (six jumpers) can dispose of this difficulty.
For anyone interested in more than one-voice music making with pulse waves, MTU's digital-to-analog converter is an excellent buy. If you don't happen to have the $50 this unit is well worth, there is another answer to "making chords," that is, if you don't care about filtering, but do like to put (very) simple circuits together, build Jim Butterfield's "Poor Man's D/A Converter" (PET Gazette, Spring 1979). I did—for $5 in parts on the back of a user port edge connector (the connector is half of the $5). It works quite satisfactorily, even without 1% resistors—using the Petunia software and external amplifier/speaker.
"CB2 sound" is a single pitch, 5-volt pulse wave available at the CB2 pin on the user port. To hear it without the MTU DAC, attach a live lead to the CB2 pin on the user port (see Commodore manual), and another to one of the ground connections on the same port. Do this through an edge connector. Attach the other ends to the line level (auxiliary) input of an amplifier/speaker. See also reference in this article to "Poor Man's D/A Convertor," which handles "CB2 sound."