From: http://support.apple.com/kb/TA40737

For reading data, the cassette recorder uses a more complicated input circuit
consisting of a 741 operational amplifier configured as a zero crossing
detector. Zero crossing detection means that whenever the voltage at the input
jack goes from positive to negative (or negative to positive) the output of
the amplifier switches from a 1 to a 0 (or 0 to 1). The detector is accessed
by any read to address $C060. The sign bit (most significant bit) of the byte
read reflects the detector status. The read routines continually EXCLUSIVE
ORs this bit with the value most recently read to detect a change in state.
The amount of time required to change state indicates the incoming frequency
which then is used to determine if a one or a zero has been received. After
detecting the first zero crossing at the start of the header, the read routine
uses HEADR to generate a 3.5 delay, and then the read routine waits for the
sync bit. After HEADR generates the synchronous bit, the read routine reads
the data and puts it in the specified memory range.

In using the cassette interface to either read or write, all you need do is
specify an address range and execute the read or write subroutine. The
address range is stored in four bytes, two for the first address to be saved
and two for the last to be saved. In both cases the least significant byte is
first.

Commanding the cassette interface:

1. from the monitor:
If the start is $800 and the end is $9FF, then
800.9FFW will write the data to the cassette and
800.9FFR will retrieve it.

2. from machine language:
Again, if the start is $800 and the end is $9FF then store the address
range,

LDA #$00
STA $3C starting address low
LDA #$08
STA $3D starting address high
LDA #$FF
STA $3E ending address low
LDA #$09
STA $3F ending address high
JSR $FEDC write to block to tape

The JSR $FEDC will write to the cassette; JSR $FEFD will read from the
cassette.

3. from BASIC:
First set up the address range. If S = the start and E = the end then from
integer BASIC,

POKE 60,S MOD 256
POKE 61,S / 256
POKE 62,E MOD 256
POKE 63,E / 256

4. from APPLESOFT,

POKE 60,S - INT(S / 256) * 256
POKE 61,S / 256
POKE 62,E - INT(E / 256) * 256
POKE 63,E / 256

Then, to write out to cassette, use CALL -307; to read in from the cassette,
use CALL -259.