What the improved blue box did was to continuously transmit a tone of some other frequency (e.g., 600 Hz) as a guard tone whenever it was not sending a System 4 signal. This guard tone drowned out the echoed acknowledgment signals so that only the blue box-transmitted digits were heard by the digit-receiving circuits at the far end.
What the improved blue box did was to continuously transmit a tone of some othe.r frequency (eg, 600 Hz) as a guard tone whenever it was not sending a System 4 signal. This guard tone drowned out the .echoed acknowledgment signals so that only the blue box-transmitted digits were heard by the digit-receiving circuits at the far end.