Digital information system, digital audio signal processor and signal converter

In a digital information system comprising a digital signal source and a memory card connected with the digital signal source in receiving/delivery of a digital signal and having a memory for storing a specified digital signal, said memory card reproducing a digital signal stored independently, said memory card with playback function having a secondary battery built therein, said secondary battery being charged by a power supply on the terminal device side when connected with the terminal device.

In a digital information system comprising a digital signal source and a memory card with playback function connected with the digital signal source in receiving/delivery of a digital signal, said memory card having a memory for receiving and storing a specified digital signal and reproducing a digital signal stored independently, said memory storing and controlling the digital signal, a connectors for receiving/delivery of a signal from an external source and an operating section for controlling the playback.

In a digital information system comprising a digital signal source and a memory card with playback function connected to the digital signal source in receiving/delivery of the digital signal, said memory card having a memory for receiving and storing a specified digital signal, said memory card reproducing a digital signal stored independently, wherein said memory has a security function of performing selected one of the operations of inverting at least one-bit digital signal of the input and/or output section of the memory in accordance with selected one of a password and a password coincidence detection signal and replacing the digital signal with another bit thereby to prevent the user from reproducing a correct digital signal.

A digital information system comprising a digital signal source and a memory card with playback function connected with the digital signal source for receiving a specified digital signal, storing the same digital signal in a memory and reproducing the stored digital signal independently,

• said system further comprising means for performing selected one of the operations of compressing and extending the information amount in comparison with the original information amount of the digital signal and noise-neglecting means for removing noises.

A signal converter comprising:

• a down counter for counting reference time pulses in response to a digital input signal supplied at a predetermined period corresponding to the maximum value of the digital signal, and
• a digital circuit for forming a pulse corresponding to the operation time of the down counter,
• thereby producing a pulse width modulated signal corresponding to the digital input signal.

In a digital audio signal playback circuit comprising a memory for storing a digital audio signal, means for reading the digital audio signal from the memory independently, a digital-to-analog converter for converting the digital audio signal to an analog audio signal, a low-pass filter, and an amplifier for amplifying a signal inputted thereto through the low-pass filter,

• a one-chip integrated circuit comprising a digital-to-analog converter, a low-pass filter, an amplifier, a controller and an interface section except for the memory.