Features & Specifications
Channels 1 (mono)
Delay 0.2-34s with increments starting at 50ms
Sample Rate 10-40kHz (see Table 1)
Voltage Resolution 12 bits
Input Sensitivity 200mV RMS
Signal-to-Noise Ratio around -70dB
Controls delay and volume adjustment (infrared remote)
Power Supply 9V DC 150mA
Lots of readers have asked us for this project. They hate the commentary on TV sports broadcasts and the same comment goes for the adverts. But if they listen to radio commentary instead, they hear the score change before they see it on the screen. Our Sports Sync project fixes that.
It lets you to delay the radio from 0.2 seconds up to 30 seconds or more, in small increments. So it’s perfect for matching up the sound and the picture.
A universal remote control is used to operate the device and the controls the Delay, Volume & Mute functions.
The rear panel of the device also carries an Output Volume control, along with an Input Gain Control and RCA input and output sockets. Three LEDs on the front panel indicate the device status: Power, Activity and Clipping.
The obvious way to provide an audio delay is to convert the sound from the radio to a digital stream, store it in a memory buffer and then convert it back to analog audio later, ie, build a digital audio delay. By controlling how much of the memory buffer is used, we control the length of the delay.
The required memory buffer is quite large – larger than the amount of internal RAM (random access memory) available in any microcontroller. Even the new PIC32 series micros have an upper limit of 128KB of RAM which at a measly sampling rate of 16kHz and a low 8-bit voltage resolution is only enough to buffer about eight seconds worth of audio.
We want a longer maximum delay and better audio quality. So we need an external RAM chip for the microcontroller.
We considered the idea of adapting the SD Card Music & Speech Recorder/Player (SILICON CHIP, August 2009) for this purpose but it uses flash memory for storage and that is not suitable for this task. For this application, the memory is constantly being written but flash wears out after a fixed number of write cycles. In some cases the number of write cycles can be quite low so it is really only suitable for data storage.
RAM, on the other hand, can be written as much as necessary without any risk of failure.
The basic design is shown in the block diagram of Fig.1. The radio is connected to CON2 and its audio output is amplified and biased to suit the requirements of the analog-to-digital converter (ADC) in microcontroller IC1. The sound is then digitised and stored in a 512KB static RAM (SRAM) chip (IC2). It is later retrieved and played back, then passed through some filtering and a volume control before being sent to output connector CON3.
The incoming audio is also applied to a clip detector circuit consisting of a window comparator, pulse stretcher and clip LED. This indicates whether the audio gain is too high for the ADC so that the gain can be set to the optimal level.
The circuit also incorporates an infrared receiver (IRD1) which is connected to the microcontroller, so it can pick up remote control signals, providing delay and volume adjustment. An 8MHz crystal oscillator ensures that the delay and sampling rate are accurate and stable. Two additional LEDs (LED2 and LED3) provide feedback on the audio buffering state and infrared activity.