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FAQ License free STL
2. Which distance can be reached?
3. What does the link cost?
4. What's so special about the Octennas?
5. Which coax cables can be used?
6. How high should the antennas be mounted?
7. Which audio quality does the link deliver?
8. What is the audio delay?
9. What is the 'Forward Error Correction' or FEC?
10. What can we do to obtain the same audio delay at the main FM transmitter site?
11. How long does it take after power up for a STAR1800 to deliver audio?
12. What does the link do with my sound processing?
13. Does the link transmit the entire MPX signal?
14. Are there licensing aspects with regard to the audio encoding?
15. Is the link absolutely free of licences and free of taxes?
16. Isn't this the frequency of the GSM 'high' band?
17. Doesn't the GSM 'high' band disturb this audio link?
18. Are there other sources of disturbance?
19. Is the link influenced by lightning?
20. What if the receiving signal is just too weak?
21. Can one transmitter feed more than one receiving site?
22. Can the digital system be used at a different frequency?
23. What is the RF bandwidth?
24. How can a spectrum authority measure the transmitted power(EIRP)?
25. What can be heard at the transmitting frequency with an analogue STL receiver or spectrum analyser?
1. How can we bridge 15km with so little power (0.02W = 20mW)?
An analogue stereo MPX Studio-Transmitter-Link (STL) needs approximately -58dBm (49dBuV) at the antenna connection to reach an audio S/N of 60dB. For a link at 1.5GHz with 'line of sight' and good parabolic dishes or yagi's (18dBi) and a coaxloss of 6dB, this means that 90W (EIRP) is needed. For instance a 6W transmitter.
The digital STAR1800 only needs –109dBm (-2dBuV) at the antenna connection to deliver perfect audio. A fading margin of 7dB is sufficient, so the link goal is only –102dBm (5dBuV). This is 44dB's less than for the analogue STL! This means that the transmitting power may be 44dB smaller. So instead of an EIRP of 90W we have enough with 3.6mW.
By using active antennas, the loss of the coax cables is completely neutralised. This means smaller flat panel Octennas can be used (13dBi).
2. Which distance can be reached?
If there is really 'line of sight', a distance up to 33km is possible. Because the radio waves (even at 1800MHz) are still somewhat refracted, this distance is also possible even if there is (just) no optical sight.
If there is no 'line of sight', but the antennas can be placed as high as possible (at least 10m high), then a distance of 5km is feasible.
If there is doubt, a test needs to be done. If an existing STL is substituted then the old receiving level can be recalculated towards the new system.
The radio propagation at 1.8GHz is completely comparable to the propagation of existing 1.5GHz links.
It's also possible for us to do a test with a small helium balloon if one of the antenna masts isn't present yet.
3. What does the link cost?
Price level of the complete set STAT/STAR1800 with Octennas, is a bit cheaper than conventional analogue audiolinks (STLs). This became possible, despite the higher complexity of the electronics, by using the latest electronic components and embedded software.
Zero after-costs: in contrast to conventional STLs or internet solutions, there is no yearly or monthly fee.
4. What's so special about the Octennas?
The Octennas are compact flat panel antennas. The gain is +13dBi. There were specially designed for a very good front/back and have minimal sidelobes. The Octenna delivers a stable signal even under rain, ice or snow. The Octennas contain a minimum of electronics, to neutralise the coax cable losses and to comply with the definition of 'integral antenna' of the EN301 357.
TX Octenna: Contains a small amplifier stage and bandpass and low pass filters. The radiated power is measured, digitised and send via the coax cable to the indoor unit. A digital stabilisation system makes sure that the EIRP is always at the preset level (max 20mW). The coax cable is a 7mm good satellite coax for lengths up to 45m, or a 10mm up to 70m (maximum 14dB loss in total).RX Octenna: Contains bandfilters and LNA's to obtain an extremely low noise figure.
5. Which coax cables can be used?
75 Ohm coaxes are used. The cheapest ones are good quality satellite coaxes. Maximum attenuation may be 14dB. Even then, there's no degradation of sensitivity.A good diameter 7mm coax has an attenuation of 30dB/100m at 1800MHz, so 45m is feasible.
A good 10mm has an attenuation of 20dB/100m at 1800MHz, so 70m is feasible.
The attenuation of the transmit coax is automatically compensated.
The attenuation of the receive coax must be fed in via the display of the indoor unit, to keep the dynamic range maximum.
The antennas and the indoor units expect a (satellite) F-connector.
6. How high should the antennas be mounted?
As high as possible. This will give a real 'line of sight' link and thus a large distance can be reached. For the RX Octenna the local possible sources of disturbance will be attenuated more.7. Which audio quality does the link deliver?
The chosen Vorbis (ref Ogg Vorbis) audio encoding is found to be superior to MPEG4/ AAC by many audiophiles. A high audio bit rate of approximately 180Kbit/s is used.The audio quality is thus very good.
8. What is the audio delay?
The audio delay is about 2,5s.The delay comes from the audio encoding, Forward Error Correction (FEC) & interleaving, receiver de-interleaving & FEC, receiver buffer fifo.
9. What is the 'Forward Error Correction' or FEC?
At the transmitter side, a large number of extra check bits are added. At the receiving side, the check bits are compared to locally generated checkbits. If there were bit faults, these are corrected by an iterative process in which all databits of a complete frame are corrected up to 20 times until no faults remain. This all happens extremely fast, well before a new frame is received.The applied FEC system is practically the most advanced which exists, which is normally only used for modern satellite links.
The application of this FEC, together with the active antennas results in the extremely good sensitivity of –109dBm (–2dBuV).
10. What can we do to obtain the same audio delay at the main FM transmitter site?
If at the studio, there is already a FM band transmitter, and this is directly modulated with the studio audio, then it has 2,5s less delay than the repeater sites who work via the STAR/STAT1800 audio link. To correct this, an extra STAR1800 can be placed at the main FM transmitter, or a special STAT1800 can be delivered with an integrated decoder so that the local delay also becomes 2,5s.11. How long does it take after power up for a STAR1800 to deliver audio?
In contrast with some satellite systems, this happens very fast: after 1,5s the audio arrives.12. What does the link do with my sound processing?
As for all types of audio links, the near DC highpass frequency response is crucial (the lower the better). With the STAR/STAT1800 this starts at about 10Hz. For practically all conventional analogue STL's this is too high (e.g. 15 or 20Hz), except for our STAT/STAR1500.
A second fact is that with all systems which employ subband encoding like MPEG, AAC, Vorbis .. the output amplitude is less fixed than the original. This is due to the limited resolution which subbands can get after the psycho acoustic software.
In case of the STAR/STAT1800, higher mentioned effects are smaller than for other systems.If the signal was limited to 100% peaklevel by a good audio processor, than at the receiving side, the audio peaks can reach 120% (20% overshoot).
This is acceptable in most cases and then an ordinary stereo FM transmitter can be used.
- use a STAR1800 with internal stereo/RDS coder
- A stereo coder with clipping.
- An audio processor with built-in stereo coder.
It's best to split the soundprocessing in two: the most complex soundprocessing before the STAT1800 and after the STAR1800 only a final clipping device and stereo coder.
Alternatively, one can do the complete soundprocessing after the STAR1800. But in this case it's possible that some musical content that was supposed to be inaudible by the psycho acoustic model is afterwards accentuated by the soundprocessor.
13. Does the link transmit the entire MPX signal?
No (although...)The link only transmits high quality left and right audio. Pilot tone and RDS are not. RS232 data and a read-in contact are also transmitted.
Following possibilities exist to obtain a MPX and RDS signal:
- Best solution: use a STAR1800 with internal stereo/RDS coder option.
- A stereo FM transmitter with separate RDS coder.
- An external stereo coder with or without internal RDS coder.
If one uses the optional delay equalisation in the STAT1800 transmitter, the same story applies.
The link can send serial RS232 data of 9600 baud. This RS232 can then be used to update the remote RDS coder.
The link also sends one 'read-in contact' and copies that towards a relay output in the STAR1800 (this can be used e.g. for the activation of the TA flag).
14. Are there licensing aspects with regard to the audio encoding?
No. In contrast to MPEG or AAC, the use of Vorbis is licence free.
15. Is the link absolutely free of licences and free of taxes?
Yes.
The STAT/STAR1800 audio link, uses the SRD norm 'wireless audio links' EN301 357 according annex 13C of Rec70-03. SRD (Short Range Devices) is a somewhat unfortunately chosen word: it only means that the equipment transmits with little power, that it mustn't cause interference and that it cannot claim protection from interference. The range isn't limited, but is short for the majority of equipment due to the low power.
The SRD standard has the advantage that the use of the equipment is completely licence free and that it doesn't has to be notified.
To get the licence free aspect 100% clearly and on paper, we did a voluntary notification of the system towards the Belgian spectrum authority BIPT. Because the word 'Short' raised some concerns (our link reaches 30km), the BIPT has presented our complete system and discussed it within the 51st SRD Maintenance Group with a power point presentation of us, which clearly stated:
- audio link for FM radio stations
- antennas in a mast
- all disputable points of the EN301 357
- arguments in regard to no extra spectrum loading
"It was concluded that as long as compliance with the essential requirements" . "was demonstrated, e.g. by means of compliance to the harmonized European standard, there would be no reason to prohibit this outdoor application." (extract from 'Minutes of the 51st SRD/MG meeting')
Or as the BIPT wrote to us:
'As agreed, we consulted the administrations of the other member states during the SRDMG (51se meeting) held at Copenhagen at January 12 - 14 2011.
None of them had objections against bringing the equipment on the market, provided that there is compliance with following technical requirements namely; it has to use the band 1795 - 1800 MHz with a maximal e.i.r.p. of 20 mW (13 dBm).' (translated extract out of email from BIPT)
'The use of the equipment is licence free as long as above mentioned technical specifications are fulfilled.' (translated extract notification confirmation document BIPT)
This all means that at European level, our audio link can be used without licence or notification:
Firstly the Rec70-03 and the harmonized norm EN301 357 state this, secondly all spectrum authorities clearly stated that there is no reason to prohibit it, thirdly there is a precedent in Belgium.
In a few member states it may not be used: Austria, France, Ireland, U.K., Italy and Slovakia.
In Finland, the use is limited.
In The Netherlands the frequency band is larger: 1785-1804,8 MHz.
See also the "Agentschap Telecom" documents on the Dutch version of the website.
16. Isn't this the frequency of the GSM 'high' band?
The GSM system in Europe works in the 900MHz (low band) and on 1800MHz (high band).In the high band the GSM phones use 1710 till 1785MHz to transmit, and the basestations transmit on 1805 till 1880MHz. The free band in between for the low power audio link applications is thus nothing else than the guard band of the GSM system, which could not have been used for anything else than low power devices.
17. Doesn't the GSM 'high' band disturb this audio link?
The RX Octenna and STAR1800 receiver form a 'large dynamic range receiver': No AGC is used, the receiver can receive very weak signals, while at a nearby frequency there is a powerful signal.A GSM basestation working in the 'high' band must not be closer than 200m, in the main direction of the RX Octenna and the TX Octenna. However, if a GSM1800 base station is very nearby, we can deliver a very sharp band filter to suppress the GSM band. So even in this case, an operational link can be set up.
A GSM phone must not be closer than 28m in the main direction of the RX Octenna. Because the RX Octenna is normally at least 35m above ground, this is no problem.
Most times, mounting an Octenna in a tower where GSM1800MHz antennas are already in place is possible. However, we will have to check, calculate or measure the feasibility.
18. Are there other sources of disturbance?
1) Other short range devices (SRD) working in the same band:These are normally placed indoors. Their maximum EIRP of 20mW will leave the building highly attenuated (probably less then 0.2mW). At low ground level there are a lot of important losses (trees etc. e.g. 10dB). The RX Octenna is normally placed at a height of at least 35m. Due to the directionality of the antenna (disturbing-) signals closer than 100m will be strongly attenuated. This all means that SRD at 100m will only give a maximum of +25dBuV and that a SRD at 1km distance will give a maximum of +5dBuV. All this, only in the main direction of the Octenna.
Conclusion: It may be that SRDs at a distance between 100m and 1 km of the receiver will block the useful signal. Emigrating to another frequency channel is then the solution.
The disturbance will be all or nothing: either the link works perfectly, or the link is in complete silence.
The 1800MHz band is at present not used. Because of the high frequency it is not easy (read: expensive) to deliver quality audio in a conventional analogue (FM) way. Only now are some microphone manufacturers designing mikes for this band.
Because of the high frequency there is practically no extra noise. The only 'man made noise' is from old timers: an antique car or motorcycle without any capacitor generates short repetitive disturbance pulses who reach into the GHz band. The link does has no problem with this: firstly is the RX Octenna placed high (e.g. 35m) and so the weak disturbance signal must first travel 100m, secondly the Forward Error Correction and interleaving will correct any bitfaults.
19. Is the link influenced by lightning?
Not at all. Lightning generates very little at these high frequencies. The few bits which are erroneous will be corrected by the FEC. All (internal) antenna parts of the Octenna are grounded so that no static electricity can be build up.20. What if the receiving signal is just too weak?
If both antennas cannot be placed higher, then the RX Octenna can be replaced by either a double RX Octenna (5.5dB more passive gain) or by the 'RX Grid Parabola' (10dB more passive gain). This means that a marginal signal becomes a perfect signal. Of course the opening angle of the antenna is smaller, so they have to be pointed more accurately. This smaller opening angle will of course also better suppress local disturbances.21. Can one transmitter feed more than one receiving site?
Yes it can, a TX Octenna is then altered so that it can feed a second Octenna with the same legal EIRP power.This will be studied and produced according to the distances and needs.
22. Can the digital system be used at a different frequency?
Yes, perfectly.We can deliver complete systems for e.g. the 1500MHz band, with which either very long distances can be bridged, or 'difficult' path links can be made perfect.
Other frequency bands are also possible e.g. 450 to 2400 MHz (if one has a licence for such a frequency).
It is also possible to order the audio modems (TX modem and RX modem) without RF units.
The developed audio modems deliver a MPX alike signal, which must be modulated in FM at the RF carrier, just like with a conventional analogue STL. The result is a GFSK signal (gaussian filtered FSK), which, just like any FM transmission, can be transmitted without linear amplifier chains. The STL equipment must be able, in this case, to transmit/receive a MPX signal up to 120kHz without too much distortion.
23. What is the RF bandwidth?
The useful bandwidth is 300kHz (98,7% of total power).Bandwidth according to EN301 357 masks is 480kHz. This is no problem because a bandwidth up to 1200kHz is allowed.
The bandwidth can be set at a lower level (during production), but then the sensitivity of the system will be approximately 6dB less.
24. How can a spectrum authority measure the transmitted power (EIRP)?
Because the TX Octenna contains a small amplifier stage and filters and this is all together with the antenna sealed in the flat panel housing, it's impossible to measure at the antenna terminals. The authority can only perform a field measurement at 200m to 1000m distance, as long as they have free sight, to calculate the EIRP power.The system STAT/STAR1800 & Octennas is designed in such a way that the EIRP power is limited to the legal 20mW. This is confirmed by the 'Declaration of Conformity'.