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A High-Quality DAB+/FM Tuner, Pt.1

We know you've been waiting for it and after more than a year in development, we are delighted to present this world-first high performance DAB+/FM stereo tuner design. It has all the bells and whistles of DAB+ in a sleek pro-quality case, with all features accessible by remote control.

by Mauro Grassi

Features & Specifications

FEATURES

• FM/DAB/DAB+ Radio (VHF Band 2 & 3, UHF L-Band)
• FM RDS (Radio Data Service) and DAB+ DLS (Dynamic Label Segment) display to show text information • 10 FM station presets
• 10 DAB/DAB+ station presets
• Time and date display
• Infrared remote control
• 160 x 80 pixel graphics white-on-blue backlit LCD
• RCA stereo output and S/PDIF TOSLINK digital output
• Can play WAV files from an MMC/SD/SDHC memory card

SPECIFICATIONS

THD+N: 0.09% mono, 0.13% stereo
Signal-To-Noise Ratio (SNR): 71dB mono, 60dB stereo
30dB Quieting: 23dBf/4µV mono & stereo
50dB Quieting: 31dBf/10µV mono; 41dBf/30µV stereo

There are lots of of DAB+ radios out there but most are little mantel-style sets with limited features and tinny sound – far below the performance that DAB+ is capable of. Make no mistake – this SILICON CHIP design will extract the very best sound quality possible out of every DAB+ broadcast signal, regardless of the sampling rate used. As well, it is also a very fine FM stereo tuner so you can listen to the FM versions of the programs – and often get even better signal quality from the analog broadcast.

Our design is based on the renowned Venice 7 DAB+/FM radio module from leading developer Frontier Silicon Co in the UK. This module is widely used in many, if not most, DAB+ radios but as we should reiterate, those designs don’t do justice to it.

By the way, the Venice 7 module is fully assembled so you don’t have to worry about soldering tiny state-of-the-art surface-mount chips – you just clip it into the main circuit board. This is driven by a dsPIC33FJ256GP506 microcontroller from Microchip Inc and this provides all the fancy display features.

Housed in a handsome case, the DAB+/FM Tuner will match in well with other hifi equipment. The front panel controls have been kept to a minimum, since all the features can be accessed using an infrared remote control. A blue ACKnowledge LED flashes to indicate when remote control signals are being received.

The main highlight is the large backlit LCD panel which displays white text and graphics on a blue background. The display resolution is 160 x 80 pixels.

This LCD shows all the station information from DAB+ stations and also from the FM stations when RDS (Radio Data Service) is being used.

As well, the SILICON CHIP DAB+/FM Tuner will allow you to play your favourite music via WAV files stored on a memory card. Just plug it into the front panel slot. So if you don’t like what is on offer from the radio channels at the moment, you can play your own selection.

There are six front-panel buttons for station presets and two more for mode and menu selection. Apart from the power switch on the lefthand side and the blue ACKnowledge LED, there is no other clutter on the front panel except for a rotary encoder. This is controlled by a large knob and is used to select stations and menu features.

Numerous features, to be described later, are available via the infrared control. For example, there are 10 DAB+ station presets and 10 FM station presets and heaps of menu features which you can set or leave as defaults.

The rear panel carries sockets for the antenna input and analog stereo outputs (RCA phono). In addition, there’s a TOSLINK (optical) digital output, a power socket and a DB9 serial socket to allow the firmware to be updated, if required. Power comes from an external 9V AC 500mA plugpack, so there’s no mains wiring to worry about.

All the circuitry is carried on two PC boards: a main board and a front-panel display board. The main board carries the Venice 7 DAB+/FM module, the dsPIC33FJ256GP506 microcontroller, the analog output circuitry, the TOSLINK transmitter and the power supply components, while the display board carries the LCD, the pushbutton switches and the rotary encoder.

As shown in the photos, the main board mounts in the bottom of the chassis while the display board sits vertically behind the front panel. The two boards are linked together via a multi-way ribbon cable.

Click for larger image
Fig.1 the main board circuit can be split into five parts: the Venice 7 DAB+/FM receiver module, a dsPIC33FJ256GP506 microcontroller (IC1), a TOSLINK transmitter, a stereo analog output circuit (IC3, IC4 & IC5) and a power supply based on 3-terminal regulators REG1-REG5. Power comes from an external 9VAC plugpack supply.

Circuit description

Refer now to Fig.1 for the main board circuit. It can be split into five parts: the Venice 7 DAB+/FM module, the dsPIC33FJ256GP506 microcontroller, a TOSLINK transmitter, a stereo analog output stage and a power supply section.

The Venice 7 module from Frontier Silicon contains all the RF and digital circuitry necessary to decode DAB+ and FM stereo multiplex signals. It produces an S/PDIF digital audio signal stream and stereo audio signal outputs when it is tuned to a station. An input is provided for a 75Ω antenna cable and the unit is connected to the main PC board via two pin headers, one 30-way and the other 14-way.

The Venice 7 receiver module is manufactured in two versions, either as a slave or master module. Our design uses the slave version and this is controlled by commands from the dsPIC33FJ256GP506 micro (IC1) via the serial port. This serial port consists of two lines at pins 15 & 16 of the receiver module (URx & UTx) and these are driven via pins 31 & 32 of the dsPIC microcontroller.

The serial port runs at 115,200 bps and uses an 8-bit word with one stop bit and no parity. The microcontroller sends commands to the Venice 7 module and receives data back from it. Error detection is accomplished using a checksum appended to each packet sent.

The Venice 7 receiver module is also connected to the microcontroller’s I2C peripheral, ie, the SDA & SCL signals at pins 36 & 37 respectively. This is a low-speed serial bus requiring only two lines that can be used to communicate with many different devices. The two lines are pulled up by two 4.7kΩ resistors. However, while this connection is there, it is not currently used by the firmware.

The receiver module is reset by bringing the nPOR line (pin 19) low. This line is driven by the microcontroller using a digital I/O pin configured as an output (RB15 at pin 30) via a 220Ω resistor. Note that the line is normally pulled high via a 10kΩ resistor to the +3.3V rail, so that the receiver module is not also reset when the microcontroller is reset.

The 220Ω resistor is used for current limiting. The Venice 7 module also pulls its nPOR pin low on start-up and although the firmware is designed to cater for this, this resistor protects both pins in case the module unexpectedly resets (asynchronously to the microcontroller).

I2S interface

Another four lines are used to access the I2S (Inter-IC Sound) interface of the Venice 7 module. These are MCLK (Master Clock, pin 24), SCLK (Shift Clock, pin 26), FS (Frame Sync, pin 25) and D (Data, pin 27).

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