N900 FM radio transmitter
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The SI4713 chip is used to transmit FM radio in the range 76-108MHz from a digital or analog audio input. | The SI4713 chip is used to transmit FM radio in the range 76-108MHz from a digital or analog audio input. | ||
| - | It can also recieve the signal level on the tuned frequency. | + | It can also recieve the signal level on the tuned frequency. This enables transmitter application to scan all frequencies, checking for vacant ones, and suggesting empty frequencies to the user for selection as transmission frequencies. |
| - | This enables transmitter application to scan all frequencies, checking for vacant ones, and suggesting empty frequencies to the user for selection as transmission frequencies. | + | |
| - | The | + | The [[Nokia N900|N900]] controls the FM transmitter through the I2C bus, and supplies analog audio to the transmitter. |
The FM signal is transmitted from a loop antenna built into the case, and connected to the PCB with clips. | The FM signal is transmitted from a loop antenna built into the case, and connected to the PCB with clips. | ||
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The BBC appreciates the advantage in principle of authorising for licence-exempt use low-power Band II transmitters constructed strictly to a common European type-approval regime and ‘CE’ marked. | The BBC appreciates the advantage in principle of authorising for licence-exempt use low-power Band II transmitters constructed strictly to a common European type-approval regime and ‘CE’ marked. | ||
| - | However, the chosen maximum ERP of 50 nW is neither sufficiently small to avoid creating interference to broadcast reception nor sufficiently large to guarantee | + | However, the chosen maximum ERP of 50 nW is neither sufficiently small to avoid creating interference to broadcast reception nor sufficiently large to guarantee that such ‘Micro’ FM transmitters will actually work as intended in areas where Band II is utilised intensively, such as London." |
| - | that such ‘Micro’ FM transmitters will actually work as intended in areas where Band II is utilised intensively, such as London." | + | |
Revision as of 10:18, 15 April 2010
The SI4713 chip is used to transmit FM radio in the range 76-108MHz from a digital or analog audio input.
It can also recieve the signal level on the tuned frequency. This enables transmitter application to scan all frequencies, checking for vacant ones, and suggesting empty frequencies to the user for selection as transmission frequencies.
The N900 controls the FM transmitter through the I2C bus, and supplies analog audio to the transmitter.
The FM signal is transmitted from a loop antenna built into the case, and connected to the PCB with clips.
https://www.silabs.com/products/audiovideo/fmtransmitters/Pages/Si471213.aspx
The documentation for the driver, along with some information on the chip is at http://www.mjmwired.net/kernel/Documentation/video4linux/si4713.txt
The package FM boost maximises the transmitter power. In some cases, this may improve reception. In others, the transmitter will already be set to highest output.
Low power transmitters that are license compliant - do not exceed the regulations - are inherently short range devices and will have poor performance in many desired usecases simply due to the regulatory limit on power usage.
In the US - the limit is aproximately 15nW, in the UK, 50nW.
The OFCOM document lists ranges to a good reciever with a fully extended antenna as 4m for 15nW and 8m for 50nW. (to achieve 'noiseless' reception)
This document from OFCOM (UK licensing agency) outlines test results for 50nW devices) and this is a BBC response to the OFCOM proposal to legalise them in the UK. Quoting the second:
" The BBC appreciates the advantage in principle of authorising for licence-exempt use low-power Band II transmitters constructed strictly to a common European type-approval regime and ‘CE’ marked.
However, the chosen maximum ERP of 50 nW is neither sufficiently small to avoid creating interference to broadcast reception nor sufficiently large to guarantee that such ‘Micro’ FM transmitters will actually work as intended in areas where Band II is utilised intensively, such as London."