The programs for C64 were stored, as well as on cartridges and on floppy disks, on the magnetic tapes of the audio cassettes (the same ones on which it came recorded music): this media had the advantage of being able to hold a good amount of data and being inexpensive. Even devices for playing the tapes were very cheap, especially when compared to expensive disk drives. The diffusion of these supports was facilitated especially by the ease of duplication, allowed by some ad hoc devices (the so – called duplicators) that from normal (but expensive and less reliable) double registration plates. Use the cassettes though it was not always easy: since it is “binary” data, the failure (or not correct) reading even a single bit could compromise the loading. In this case, it was necessary to clean the head of the recorder if not to change its alignment … and then maybe find out what the problem was due to the malfunction of the belt drive mechanism, which often it caused the spill (and rupture, in extreme cases but unfortunately not very rare) of the same. Tapes also tend to get damaged over time – like all magnetic media – especially if not carefully preserved: for this reason it is very important to preserve them as soon as possible, by digitizing them the content through the “dumping” procedure. The most artisanal way (but no longer simple) to perform a “dump” is the so-called “audio method”, which consists in transferring the audio content of the cassette to a computer and then processing it with special programs.
All you need is an audio recorder, which must be connected to the sound card of the computer in the “Line In” socket (or, failing that, in the one for the microphone). To record and process the audio track, a free editing program such as Audacity (https://www.audacityteam.org/) is sufficient. Before recording, it is necessary to adjust the input volume of the signal: if the recorder is equipped with volume adjustment, it is good to act on it first, bringing the output level almost to the maximum; through the Audacity monitor (or the editing program that is more congenial to you) you will then have to adjust the input level so that it does not distort. (fig. 01)
The parameters to be set for recording are the frequency of sampling (44.100hz or 48.000hz are more than enough) and the resolution (16 bit). Before starting the acquisition, it is advisable to rewind the tape a few times, and wipe the reading head with a cotton swab hydrophilic soaked in a little alcohol.
The audio signal is mono, but if the recorder has stereo output (I assume that the cable is stereo on both sides) you will get two distinct channels: after completing the recording you should save it and work on one of the two channels; if there are multiple programs on the tape, you can also copy and paste them one at a time into separate files (remembering to take only one channel): usually between one program and another there are a few seconds of silence. (fig. 02)
The files thus obtained must be processed through the Audiotap program (http://wav-prg.sourceforge.net/index.html), by clicking on “Create a TAP file” and selecting “from an audio file”. If your sound card reverses the waveforms, you will also have to tick the appropriate box. The “Advanced Options” should first be left as they are (apart from the sampling frequency, which must coincide with that of the recording); if the conversion is not successful (and after excluding that it is not the inverted waveforms (fig.03) you can try to change the level of “Sensitivity”, using values around 80 for tapes not well preserved; if the tape is in good condition, a value between 10 and 20 is sufficient. Once the various choices have been made, the program gives you the possibility to give a name to the .TAP file and to choose the version of the TAP format to use (if you are dumping a tape for C64 or VIC-20 you have to use version 1, while for C16 you should use version 2.) Finally, you just have to try loading the .TAP file with an emulator, to verify its operation.
The conversion can also be done “on the fly”, by selecting the “Create a TAP file from a sound” option, but it is preferable to use the system I illustrated above: the .WAV file, in fact, proves to be invaluable in the event that the dump was not successful: first of all, listening to the audio you can understand if the tape was turning at the right speed: the tone of the initial signal must correspond to an over-acute MI. In the event that the tone is lower you can easily change the speed and intonation of the entire file through the appropriate Audacity function. If, on the other hand, the tone is not stable, you have no choice but to perform the acquisition again, perhaps after rewinding the cassette … with the hope that it is not really the cassette that is defective at the origin. It may happen, in fact, to make dozens of attempts and acquisitions, to perhaps realize that the tape is unrecoverable but fortunately the software and hardware available today are of great help, both in the manipulation of the audio signal and in the real acquisition and proper.
Like every human work, dumping also needs practice and some failures before it can be mastered with skill. Undoubtedly it is an activity, especially if done with love and dedication, capable of giving great joys and that incomparable sense of satisfaction that pays off for so much effort and work! In our activity started a few months ago, we found, among a lot of widely available software, some little gems that had been ignored pending the final oblivion.