Tapes

Before the floppy disk, there was the cassette recorder. It was an ordinary home audio cassette deck, with a microphone jack and an earphone jack. MITS sold a board that changed bytes into a sound that the deck could record, and changed the sound back into bytes. That board is the 88-ACR, and it is in this simulator.

This chapter starts with the most important use of the ACR: loading Altair 4K BASIC 3.1 as people loaded it in 1976. After that, it describes how to use a tape: mount it, move it, play it at its real speed, use audio .WAV files, and record.

The Sol-20 has its own cassette interface, with two decks and motor control that the guest can use. Everything in this chapter also works on the Sol's decks, with the unit names sol0:tape1 and sol0:tape2. The sol section of the Boards chapter gives the differences.

Load Altair 4K BASIC 3.1 by hand

An Altair owner did this every time that they wanted to use BASIC. There was nothing in ROM and no disk. Type the commands in the folder where you unzipped the package.

1. Start the built-in basic4k machine. It has an 88-ACR, and its sense switches are set for a cassette load:

$ altairsim basic4k

2. Put the cassette in:

altairsim> MOUNT acr0:tape "examples/basic4k/4K BASIC Ver 3-1.tap"

3. Enter the bootstrap:

altairsim> LOAD "examples/basic4k/LDR4K31.HEX"

The bootstrap is 20 bytes. On a real Altair, you entered it by hand on the front-panel switches, from the listing that MITS printed in its manual. You set eight switches for each byte, and pressed DEPOSIT. LOAD puts the same 20 bytes in memory.

4. Run the bootstrap from address zero:

altairsim> RUN 0

The bootstrap reads the tape, BASIC loads into memory, and BASIC starts.

5. Answer the three questions. Press Return for the first two, and type Y for the third:

``` MEMORY SIZE? TERMINAL WIDTH? WANT SIN? Y

742 BYTES FREE

ALTAIR BASIC VERSION 3.1 [FOUR-K VERSION]

OK ```

An empty answer means "all of the memory" and "the default width". WANT SIN? asks whether to use some of your 4K for trigonometry.

You are in Altair BASIC, the first product that Microsoft sold. The Worked examples chapter does the same with one command, altairsim examples/basic4k/basic4k.toml.

If a tape load stops, and the tape is mounted, check the sense switches first. The bootstrap reads them to find the device to load from. The basic4k machine sets sense = 0x80 on the front panel: A15 on, for a cassette load, and all other switches off. With a different setting, BASIC waits for bytes from a device that is not there. The fp section of the Boards chapter describes the switches.

The 88-ACR

acr is the MITS 88-ACR, an Audio Cassette Record and playback interface. It is an 88-SIO channel B with an FSK modem connected to it, as MITS built it. The guest uses an ordinary serial port, and the modem changes the bits into tones.

• The default port is 06. 06 is status and control, and 07 is data.
• It runs at 300 baud, the speed of the tape. You cannot change it.
• It has one unit, tape, because a cassette recorder has one place for a tape.

You cannot CONNECT the ACR. Its line goes to the modem on the board, and the modem goes to a cassette deck. There is no connector for a terminal on the back of an 88-ACR. The serial chapter describes the boards that take endpoints.

You put the tape in by hand. An 88-ACR cannot start, stop or rewind the tape, and the machine does not know that a tape is there. For this reason, a machine file has no key for the tape. The tape is not hardware. You put it in with MOUNT, and a startup command can type the MOUNT for you, as examples/basic4k/basic4k.toml does.

Use a tape

Put a tape in

altairsim> MOUNT acr0:tape "examples/basic4k/4K BASIC Ver 3-1.tap"

The ACR has one unit, so you can leave out the unit and the number:

altairsim> MOUNT ACR tape.bin

A Sol has two decks, so you must name the deck.

mode = play | record

altairsim> SET acr0:tape mode=record

play loads from the file, and record saves to it. A tape has one head, and it moves in one direction at a time, so mode has two values only.

The tape counter

SHOW MOUNTS and SHOW <id> give the position of the head:

altairsim> SHOW MOUNTS
  UNIT       KIND  HOLDS
  acr0:tape  tape  4K BASIC Ver 3-1.tap  00:00 / 02:28 (0%)  1/4439 bytes

The counter gives minutes and seconds into the tape, a percentage, and a byte count. For a .WAV file, the time is the recording's own time, with the leader and the silent gaps between programs, as on a real cassette counter. For this reason, a program that a manual finds "so many seconds from the start of the tape" is at that second here. A byte .TAP file has no audio, so the program calculates its time from the baud rate.

When a tape plays at its real speed (rate = real, below), the counter counts up on the console while the tape loads. Turn it off for a machine whose guest writes to the same terminal:

altairsim> MOUNT acr0:tape "tape.wav" counter=off

SET acr0:tape counter=off also works. When the counter is off, SHOW still gives the position.

WIND and REWIND: move the head

WIND <id>:<unit> <mm:ss | START | END>
REWIND <id>:<unit>

WIND (WI) moves the head to a time: mm:ss, a number of seconds, START or END. On a cassette with several programs, read the counter or a manual for the start of the next program, and wind to that time:

altairsim> WIND acr0:tape 0:05
acr0:tape: wound to 00:05 / 02:33 (3%) -- 4K BASIC Ver 3-1.wav (1 of 4439 bytes)

REWIND (REW) is the same as WIND … START. Use it to load the same tape a second time. After the guest reads a tape, the head is at the end, and a second read gets nothing.

stop: stop the tape at a time

On a tape with several programs, one program runs into the next. The stop mark is like pressing the STOP button of the recorder at a counter mark. Set it with MOUNT, or with SET at any time:

altairsim> MOUNT acr0:tape "tape.wav" stop=2:05
altairsim> SET acr0:tape stop=2:05

The line goes silent at 2:05, as at the end of the tape, and the loader does not read into the next program. To continue, move the mark forward or clear it, and play again:

altairsim> SET acr0:tape stop=5:30      (the next program ends here)
altairsim> SET acr0:tape stop=off       (play to the end of the tape)

The mark is off until you set it. SHOW shows a mark as stop @ 02:05. The mark stops playback only, and a recording writes past it. If you wind the head past a mark, the tape stays stopped there, and SHOW says why.

Speed: rate = full | real

The tape has its own clock, and by default it runs as fast as possible. With rate = full, the guest gets each byte as soon as it is ready to read the next one. A cassette that took a real Altair about 110 seconds loads in about one second.

altairsim> SET acr0:tape rate=real

rate = real plays the tape at the tape's baud rate, 300 or 1200, and a load takes as long as it took on the real machine. Use it when you want to show the wait, for example in a demonstration. It applies to playback only. A recording takes as long as the guest takes.

rate is not the processor clock. The tape and the processor had separate crystals on the real hardware, and they are separate here. SET cpu0 clock_hz=2000000 gives the processor the speed of the period, so that a game plays at its real speed. It does not change the speed of the tape. With rate = full, the guest sees no difference at any processor speed, because a polled loader only asks for the next byte, and the byte is always ready.

Audio tapes: .WAV

Most surviving Altair and Sol cassettes are audio. Somebody played a cassette into a sound card, and saved a .WAV file. You can mount one on either board.

examples/basic4k/ has 4K BASIC on an audio tape, 4K BASIC Ver 3-1.wav. It holds the program of the .tap, encoded in the audio format of the 88-ACR. The machine file basic4k-wav.toml mounts it and boots. In that folder, type:

altairsim> MOUNT acr0:tape "4K BASIC Ver 3-1.wav"
acr0:tape: mounted 4K BASIC Ver 3-1.wav
4K BASIC Ver 3-1.wav: fsk300, 4439 bytes, 0 framing errors (100.0% of frames intact)

examples/basic1/ has Altair BASIC 1.0 on an audio tape too, BASIC Ver 1-0.wav. The machine file basic1-wav.toml mounts it.

The program decodes the recording one time, when you mount it, and never while the machine runs. After that, a .WAV works as a .TAP does. The counter gives the real minutes and seconds of the recording, and the guest sees no difference.

Read the second line of the mount output. It gives the number of framing errors. A tape that decodes at 60% is noise, not a program, and the program refuses a decode below 90%. The percentage counts only the frames whose start and stop bits were in the correct place. A worn recording can keep its framing and still give wrong bytes. If a tape mounts well and the program still does not run, the recording is probably worn.

The start of the file decides the format, never its name. A .TAP file that somebody renamed .WAV is still read as bytes. A recording renamed .TAP is still decoded as audio.

extract: one .TAP file for each program

A cassette often holds several programs, with a few seconds of silence between them. extract writes each program to its own .TAP file, so that you can mount or load one program at a time:

altairsim> MOUNT acr0:tape "games.wav" extract
acr0:tape: mounted games.wav
  games-1.tap  2048 bytes
  games-2.tap  3120 bytes
2 programs extracted

The files go beside the WAV, numbered from 1. A tape with one program gives games.tap, with no number. extract=<base> sets the names, for example extract=disk1 gives disk1-1.tap. extract reads the tape and writes the files. Nothing in the machine changes.

To split a WAV that is already in the deck, use the EXTRACT command:

altairsim> EXTRACT acr0:tape

You can extract only a .WAV. The program splits at one second or more of silence. That is much longer than any gap inside a program, so no program is cut in half.

A board refuses audio that it cannot hear

examples/basic4k/ also has 4K BASIC in a modulation that the 88-ACR cannot read, 4K BASIC (Kansas City).wav. The board refuses it:

altairsim> MOUNT acr0:tape "4K BASIC (Kansas City).wav"
acr0: 4K BASIC (Kansas City).wav: this board's modem cannot hear that tape -- it carries 2400 Hz / 1200 Hz, and this board reads fsk300

Cassette audio of the period uses different tones:

• The 88-ACR uses 2400/1850 Hz FSK.
• Many archive tapes are Kansas City, 2400/1200 Hz.
• The Sol's CUTS tapes are one octave lower, 1200/600 Hz, at 1200 baud.

The decoder of the program could read all of them, but a real 88-ACR could not. Its decoder is a PLL centered at 2125 Hz, with a range of about ±100 Hz. A 1200 Hz tone is far outside that range, so the real board reads nothing from that tape. If the program decoded the tape anyway, it would give the guest data that no 88-ACR could produce. For this reason, the board tells you what the tape is, and you use a machine that can read it.

The frequencies in the message are a measurement. On a worn recording, a value can be one octave wrong. The message means "this is not my format, and this is about what is there".

Record a tape

Set the recorder to record, and run the guest. When the tape stops, the program writes the recording to the file:

altairsim> SET acr0:tape mode=record
altairsim> RUN 0                        (the guest records)
altairsim> SET acr0:tape mode=play      (the program writes the file here)

The tape stops when you set mode=play, UNMOUNT, WIND, REWIND or QUIT. On a Sol, the guest can also stop the motor. The program writes the whole file again each time.

A recording writes over the file. A mounted .WAV is recorded in the format and at the sample rate that the program found when it decoded the file. Record on a copy if you want to keep the original.

A blank tape: MOUNT … CREATE

altairsim> MOUNT acr0:tape new.wav CREATE mode=record
altairsim> MOUNT acr0:tape new.tap CREATE mode=record

A blank file has nothing to examine, so its name decides what it becomes. A .wav name makes a blank audio tape. Any other name makes a blank byte tape. The mount line tells you which one you got:

new.wav: blank fsk300 tape, ready to record
new.tap: blank raw byte tape

Give mode=record, because the deck starts in PLAY. The guest can then write to the tape, for example with CSAVE in BASIC.

A file that already exists is different. Its start decides, never its name. A .wav file that is not really a WAV is read as bytes, and a recording then puts bytes in it, not audio. Nothing can play it. If you wanted audio and the mount line says raw, this is what happened.

The leader, the tone and the level

A byte image holds no times, so the program adds the leader that a real tape needs when it writes audio. These properties of the tape unit shape the audio:

Set leader or trailer to 0 to make the file as small as possible. The archive .wav files are cut in this way, and for this reason they do not load on real hardware. Even at 0, the program writes sixteen bit times of tone at each end, because the loader finds a start bit by its edge.

The Sol's decks have shorter defaults, and one more property, rc. The sol section of the Boards chapter gives them.

A tape with several files comes back as one continuous recording. The bytes do not mark where one file ends, so the gaps between programs are not written back.

None of this applies to a .TAP file.

format: choose the format yourself

Each tape unit has a format property. The default, auto, is almost always correct.

altairsim> SET acr0:tape format=raw
altairsim> SHOW acr0

format selects how the program reads the file. It never changes the hardware. If you tell an 88-ACR to decode cuts1200, it refuses. The read-only detected property gives the format of the mounted tape.

format also selects the modulation of a blank tape from MOUNT … CREATE. A .wav name records with the board's own modem, and format selects a different one by name.

format takes effect at the next MOUNT, because the program decodes a tape when you put it in. SET names the unit, and SHOW names the board. There is no SHOW <id>:<unit>.