The Portastudio 4-Track Cassette Recorder: Below is a photo of the outside and of the inside of my Portastudio cassette recorder. Since I have no schematic for my Portastudio (does anyone have one that they can send to me?), I'm forced to figure things out by physical examination. So, like a doctor poking and prodding, let's begin...
|My Tascam Portastudio 424, Outside and Inside|
Erase Head vs Record Head: Since I assumed that my problem with the gap in my recorded loops was due to the erase head, I began my examination with finding the erase head. As you can see in the picture below, it is nicely labeled. You can also see that it is indeed physically separated from the play/record head (by about an inch). When recording (or playing), the tape passes over the erase head to be cleared prior to passing over the record head to receive the new audio. As a result, there is always a one-inch section of the tape that will contain no audio at all. This is the gap that I have been hearing in my loops.
|The Erase and Play/Record Heads on the Portastudio|
|Seeing Where the Erase and Play/Rec Heads are Wired Back to the Main Circuit Board.|
Unplug the Connector: The easiest way to defeat the erase head is, perhaps, simply to unplug it. So, that's what I tried.
|To Disable the Erase Head, Just Unplug the Big Connector|
What are the "Erase" Signals: Because I was interested, I hooked up my oscilloscope to the pins on the connector that I just unplugged. I wanted to see what signals where being sent to the erase head. The pictures below show what I found. The connector has 8 pins, and each pair seem to give similar signals (I'd assume that each pair represents a single channel on the 4-track tape). For each pair of pins, one pin showed an 87 kHz signal with an amplitude of ~134 mV RMS. The other pin showed no signal, so it might be ground. I'm assuming that the 87 kHz is the bias signal used by cassette systems as the carrier for real audio signal that is amplitude modulated on that carrier. Since this is the erase head, there should be no audio, so it should just be the 87 kHz signal. This appears to be what I see.
|Looking at the Signals on the Connector to the Erase Head.|
Looks like the 87 kHz AC bias signal (Left) and Ground (Right).
Trying it Out: So, with the erase head disconnected from the main PCB, I tried recording some audio. As a sound source, I used my Korg Monotron with its funny little ribbon controller. As you heard in the demo tracks tat the top, when I defeat the erase head (by disconnecting it from the main PCB), I successfully eliminate the gap from my looped audio. Fantastic.
|Using the Monotron Delay to Create Tracks on the Portastudio|
Visualizing the Results: In addition to listening to the demos, you can see the tracks visually in the spectrograms below. All of the tracks show recordings from the loop tape of the Monotron. The top figure shows a single loop of audio excerpted from a recording when the erase head was connected and active. Note the half-second gap at the end of the loop, which corresponds to that one-inch piece of tape between the erase head and the record head. The middle plot shows the result after I unplug the erase head. Note that the gaps is gone...the loop is continuous. This is what I hoping for.
|Spectrograms of the Same Three Audio Demos From the Soundcloud Player at the Top of this Post.|
Layering Audio in One Track: Another important effect of defeating the erase head is that nothing clears the track between recordings. So, when you go to record audio onto a track, it becomes layered on top of the audio that was already there. An example of this sound-on-sound recording is given in the third audio demo (and in the third spectrogram above). Here, I first recorded the basic warbling tone from the Monotron as already seen in my other examples. This is "Layer A". Then, onto the same track, I recorded the Monotron with wilder variations in pitch. This is "Layer B". As is clear in the audio demo and in the spectrogram, both layers are clearly there. Note that this is not a mixture that has been done electronically (ie, read the previous track, mix it with the in-coming new audio, then record the mixed audio onto a fresh track). No, this mixture is being done on the tape itself. As far as I know this is very uncommon.
Imperfect Layering: I believe that one reason why it is uncommon to mix audio onto tape this way is because the layering is far from perfect sounding. Note that as "Layer B" dives from high-pitch to low-pitch (see the blue arrow), it seems to suppress a lot of the "Layer A" sound. It's not until the pitch of "Layer B" raises a bit that the tone from "Layer A" sounds strong again. To most people, this interaction between the two layers of audio would be an undesirable trait. But, not for me...I'm purposely looking for unusual side-effects of using tape. So, to me, this kind of weird artifact is gold!
Have any of you hacked your cassette recorders? What have you done?