Apparatus for a record player distinguishing between recorded and unrecorded portions of a disc

Apparatus, including a tone arm, for use with equipment for playing a disc, said apparatus distinguishing between recorded and unrecorded portions of the disc, comprising:

• a pair of light-sensitive devices mounted on the tone arm for facing the disc, the devices being spaced across the width of the tone arm in leading the trailing positions for sensing light reflected from separate but adjacent portions of the disc, said devices individually producing an electrical signal when at least a predetermined level of reflected light is sensed;
• a circuit for uniforming the potential strength of said electrical signals produced by said light-sensitive devices including a pair of transistors each having a base, a collector and an emitter, each base being connected respectively to the output terminal of the light-sensitive devices, each collector being connected to a terminal of a like power source, and each emitter being resistively connected to a like reference source terminal;
• and
• means coupled to said light-sensitive devices through said uniforming circuit for determining the relative states of the devices as being reversed or not reversed from a predetermined relationship between the devices as to sensing or not sensing said predetermined level of light, said means comprising:
• a circuit for establishing a difference between the potentials of the signals emitted by said devices through said uniforming circuit when said predetermined light level is sensed for establishing said predetermined relationship, and a comparator, said comparator having a pair of input terminals each connected to an individual one of said light-sensitive devices through said potential difference establishing circuit and said uniforming circuit, said potential difference establishing circuit including a first resistor connected between one of the transistors and one of the input terminals of said comparator, a second resistor connected at one end to said one comparator input terminal, a third resistor connected between said second resistor and the reference source terminal, a forward diode parallelly connected to both of said first and second resistors, and a direct connection between the other transistor and the other input terminal of said comparator.