Several ideas and hardware descriptions for reading perforated music
rolls, and creating an electric signal when a hole is sensed, are
presented in this issue of MMDigest.  The three sensing categories under
discussion employ either air pressure, air flow, or light (reflected
or transmitted).

Here's a summary of the hole sensing methods which are either in use
already for transcribing music rolls or are under development at the
experimenter's workbench:


Light Sensors

Optical Line Scanner - this is a specialized video camera which scans
across the note channels and sends a video signal to the computer where
pattern recognition software identifies the center of the punched hole.
The resolution is 2048 or more pixels per line, thus a 9-per-inch music
roll of 100 channels has about 19 pixels for each note channel.  (The
total width scanned is assumed be 12 inches.)   To capture a small
(narrow) roll, like a 44-note Pianino, the camera is moved closer to the
paper and the lens focus is adjusted.  That's about all!  The biggest
advantage of this technique is its ability to view any spacing between
the channels, without changing a tracker bar or sensor array.  It also
has the future potential to capture the printed lyrics as the roll
scrolls by.

Photosensors Array - an array of light sensors is arranged in a line
just like a tracker bar; the spacing between the sensors matches the
spacing between the channels of the music roll, and there is one sensor
for each note channel.  The advantage of this technique is its simplic-
ity and reliability: there are no pneumatic components, and the elec-
tronic components are inexpensive and readily available.  Like the
optical line scanner camera, problems can arise with translucent paper,
since the hole is sensed as a change in light intensity.  (And, natur-
ally, a hole or a rip covered by transparent mending tape won't be
detected.)


Air Pressure Sensors

Pouch Switch - A traditional pneumatic pouch is fitted to an electric
switch assembly (e.g., a micro-switch) or to electric contacts or to the
shutter of a opto-interrupter switch.  One side of the pouch is exposed
to constant vacuum, the chamber on the other side connects to the track-
er bar and to a small bleed to a vacuum supply.  It's practically the
same as the valve in a player piano action.

Strain Gage Sensor - Instead of a pouch, the tracker bar and bleed are
connected to a diaphragm with a resistance bridge cemented to it.  The
deflection of the diaphragm is detected as a change in resistance.

Piston & Cylinder Sensor - A tiny opaque cylindrical piston fits loosely
in a glass tube and is allowed to move a short distance when the differ-
ential pressure changes sense.  The position of the cylinder is detected
by a photo-interrupter switch.  If the airflow over the piston is small
this device can be considered a variant of the pouch switch; if the
airflow is large then it can be considered an airflow switch (which
doesn't use an auxiliary bleed to the vacuum source).

Bubble Sensor - This novel proposal detects pressure change via the
motion of a gas bubble trapped in a capsule of liquid.  The bubble
position or motion is detected by a photo-sensor.


Airflow Sensors

Hot Wire Sensor - A tiny temperature-dependent resistance wire (therm-
istor) is placed inside an air passage and heated with an electric
current.  The temperature is dependent upon the mass air flow across the
device and is sensed as a change in resistance.  This device needs no
bleed -- all the air entering the tracker bar flows across the sensor
and into the vacuum manifold.

Robbie Rhodes