OFDM the Future of digital radiomic transmission?

OFDM the Future of digital radiomic transmission?

 18th May 2020 |  Blog |  David Harcombe


OFDM the future of digital radiomic transmission.

We all know about latency in audio circuits and of course the goal of ang audio processing is to get the latency down as much as possible.

In wired circuits this is usually a product of either the speed of light, or the processing of the particular amps and circuitry, pretty fast.

With Wireless technology you are looking at the speed of RF in atmosphere which again is about  the speed of light. So the latency usually occurs due to processing on the signal, and with digital radio mics having to encode, analogue to digital convert to RF using a baseband transmit across free space and then de-encode this can take be anywhere from 2-18mS, depending on the modulation scheme.

So where does this latency occur. Within the Modulation scheme is hidden the clue, if for example QAM-4 or QPSK modulation scheme is used then the latency occurs due to the error correction that needs to take place once all the bits have been received by the Receiver, as more than one bit is transmitted per cycle.  So if an error occurs in the transmission then the receiver has to wait for the next cycle to receive the correct bit. So if a more dense modulation scheme is used such as QAM-16 to reduce the band width then the error rates goes up according to the Nyquist theorem, as the possibility of errors across transmission increase. If the receiver has to wait for the bit to come in due to a higher chance of errors, then latency increases, as the Rx has to wait for correct complete bits to arrive before it passes them o in the signal path.

If this is the case then how can you overcome the inherent latency in narrower bandwidths. This is where OFDM or Orthogonal frequency-division multiplexing.

Slight phase changes to the carrier wave making sure as the diagram suggests, the peaks coincide with the troughs allow for the same information to be sent via multiple subcarriers allowing for the RX to pick which stream to read and hence increase the reliability and latency of the transmission, as well as, inherent resilience to narrow-band RF interference.

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OFDM the Future of digital radiomic transmission?

OFDM the Future of digital radiomic transmission?

OFDM the future of digital radiomic transmission. We all know about latency in audio circuits and of course the goal of ang audio processing is to get . . . the latency down as much as possible. In wired circuits this is usually a product of either the speed of light, or the processing of the particular amps and circuitry, pretty fast. With Wireless technology you are looking at the speed of RF in atmosphere which again is about  the speed of light. So the latency usually occurs due to processing on the signal, and with digital radio mics having to encode, analogue to digital convert to RF using a baseband transmit across free space and then de-encode this can take be anywhere from 2-18mS, depending on the modulation scheme. So where does this latency occur. Within the Modulation scheme is hidden the clue, if for example QAM-4 or QPSK modulation scheme is used then the latency occurs due to the error correction that needs to take place once all the bits have been received by the Receiver, as more than one bit is transmitted per cycle.  So if an error occurs in the transmission then the receiver has to wait for the next cycle to receive the correct bit. So if a more dense modulation scheme is used such as QAM-16 to reduce the band width then the error rates goes up according to the Nyquist theorem, as the possibility of errors across transmission increase. If the receiver has to wait for the bit to come in due to a higher chance of errors, then latency increases, as the Rx has to wait for correct complete bits to arrive before it passes them o in the signal path. If this is the case then how can you overcome the inherent latency in narrower bandwidths. This is where OFDM or Orthogonal frequency-division multiplexing. Slight phase changes to the carrier wave making sure as the diagram suggests, the peaks coincide with the troughs allow for the same information to be sent via multiple subcarriers allowing for the RX to pick which stream to read and hence increase the reliability and latency of the transmission, as well as, inherent resilience to narrow-band RF interference.
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Covid-19

Covid-19

Covid-19 it totally changing the way we conduct ourselves in the Film & TV world, social distancing means that the world of both factual and . . . non-factual TV may change for ever. So until that happens here are a few guidlines to help with the planning of factual shoots in the future. Factual TV Coronavirus working practices Background assumptions Factual TV is a very different beast when it comes to production than that of drama with smaller crews, less resources, a traditional need for individuals to mix between departments, and often a mix feed to camera or cameras as well as a  greater use and reliance on radiomics being essential not just advisory. This coupled with actuality often dictating the shooting schedule and a fast daily turnaround, gives rise to a greater possibility of cross contamination, of both personnel and equipment, between departments. It is therefore vital that certain safe practices be implemented to minimise the possibility of Coronavirus transmission, under these circumstances. These should include increased general and specific Covid-19 planning prior to the shoot, as well as certain working practices  during the shoot to minimise any crew member to coronavirus transmission as well as any production to health and safety litigation based on unsafe working practices. The overall effect of these procedures will be to avoid as much as possible cross contamination of  equipment and Personnel across multiple locations, if cross contamination between equipment is unavoidable due to the nature of the filming process, then appropriate time and resources to be allowed for the effective cleaning and preparation of this equipment, for the next shooting process to mitigate as much as possible this risk of this cross contamination. The mitigation of cross contamination by personnel, by the use of controlled spaces on set or locations as well as the limiting of access to these spaces to predesignated personnel. Outside any controlled spaces protocols of social distancing should be in place where practicable.   Prior planning A nominated kit zone for every location /new location, access to which is limited to certain predesignated personnel from each department. Can you get any kit to camera the day before and set it up such as mix feeds and timecode…? If not dedicated time for either Camera to set this up in the designated kit safe zone to avoid cross contamination with access to only 1 person for camera dept or Sound Dept. Prior decontamination of Camera attachment areas with isopropyl alcohol. Use of single use gloves for kit attachment. Any unused equipment to say in this safezone. Disinfect all radiomic equipment such as personal Lav mics pre-package all disinfected equipment which may be crossed to other depts such as timecode boxes, TX’s and batteries. All crew to arrange for possible short notice replacements with the same appropriate kit and protocols in place if any symptoms show overnight. Possible Covid-19 testing to take place prior to longer term shoots as close to shoot start as possible. Additional paid time built into schedule for effective get in and get out, and time spent decontaminating and kit preparation with appropriate safety protocols, during the shoot, close liaison with PM to action this effectively. Proof of appropriate insurance policies by both production and crew i.e. PLI and corporate liability insurance covering covid-19. Liaison with Production as to who within the production would be happy with a reused radio mic and mic capsule ( disinfected ) and who would want a dedicated pack and capsule used exclusively for and by them over the life of the production. Possible hire of additional radio mic equipment to ensure no cross contamination of radio mic effects across the life of the production.   On the day protocols if radiomics are being used.   Dedicated area at each location kept as a safe Radio mic area access to only 1 person from sound, within each social bubble. Minimal kit. Or safe area within social bubble dedicated to 1x sound personnel for micing and  Possibility of once mic’ed leaving capsule on personal for all day. Prepacked  disinfected consummables for each contributor…? For sit down Interviews linear crew prep and access to shoot area one by one, lighting, Camera and sound with appropriate stand in (ie if we know the height of the interviewee set up with stand-in from crew) Additional time given for get in and kit prep and micing/ demicing  using above procedures. Appropriate PPE to be worn Face mask whilst shooting. No cross contamination of Kit between depts. Kit from each dept to only be handled by dedicated person from that department. Dedicated HS person from Crew to monitor procedures and social distancing. If symptoms reveal themselves during shoot replacement of Crew member as soon as practicable. If lots of radio micing/demicing needs to take place, production to make sure additional time /personel are available to allow minimal contact between multiple crew members. All sound kit, which has been used on contributors to be decontaminated on set with IPA wipes, between contributor turnover and before wrap repacked ready for the next days shooting. Appropriate  resources and time to be  given each day for breaks to include hand washing and personal corona virus hygiene/ decontamination.
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The world of digital radio mics

The world of digital radio mics

Digital vs Analog radiomic Range Analogue RF 1. Analog as a compressed sine wave. Analogue radio mics have been around since the early 70’s with . . . Micron explorer radio mics being used on such films as The Shining, among others. Due to the limited bandwidth of carrier wave, most used some kind of Compander a compressor in the Tx with an Expander on the Rx in order to squeeze the audio into a 60db window for transmission. This of course was different for each manufacturer each using a proprietary compander to make their radio mic sound better than their competitor, but the system was essentially the same. Whilst the processing of this audio requires time there is always a lag or latency between the transmission of the signal and the final audio processing of the signal out of the radio mic receiver, The Carrier wave was a FM sine wave with a band width of 200khz or 175khz requiring a 400 khz gap between frequencies, however due to physics the available “clean” frequencies within a given band decreased as the number of radio mic channels increased, this is due to intermodulation. In Analog systems intermodulation is a fact of life, if two similar frequencies are combined ( a natural fact of RF signal propagation in free space) and in the same way that harmonies are produced in music. Additional frequencies are produced in the RF spectrum. These interfere with the signal that the receiver wants to lock onto and see, if the harmonic frequencies are sufficiently high in amplitude then the Receiver finds it hard to differentiate between this frequency and the one in wants to see, resulting in reduced range and audible interference in the signal. These are known as 3rd order interference products, however that is not the only story, these 3rd order can then interfere and produce harmonics with each other and the fundamental freq, producing 5th order interference products, these usually have less amplitude that the 3rd order products and are usually further away from the fundamental freq, meaning that the RX can usually operate normally with these products around. Although Ofcom allows for body worn transmitters to emit 50mw this rarely equates to the ERP, due to losses in the circuitry and antenna position etc. under good conditions at a distance of about 100ft the field strength of the signal from a 50mw transmitter is about 1000uV/m well within the sensitivity of most RX’s within the UHF spectrum any increase in frequency is accompanied by an increase in loss so the higher the Freq the greater the lose over distance. In the real world this in not the only problem with signal reception, as all RF is emitted in a 360 degree pattern from say a whip Antenna, the signal path between the TX and RX is not only direct, but is often reflected by multiple surfaces, meaning the RX often “sees” more than one signal which can obviously interfere with the clarity of the received signal. So how to avoid or mitigate against this. Diversity is a system where the signal received from a transmitted Rf source can be received from one of two seperate antenna, with either a physical separation (ie a small distance apart) or a planar separation ( ie vertical and horizontal planar separation) The separation allows for the peaks and troughs for the interefence pattern to be avoided as the RX can choose between the stronger signal at the RX antenna, whichever is stronger the RX circuitry chooses this signal and due to the small distance between peaks and troughs of a UHF signal , a few cm is usually adequate to see a radically different signal at the RX. This is antenna diversity. However this system can be improved upon by adding multiple RX circuitry to both Rf reception paths each RX path can choose between each Rx antenna, giving a total of 4 signal paths So the final AF out can choose and mix between 4 different signals allowing for even greater robustness of the final signal. Even with all this analogue radio mics are still susceptible to signal loss, and the way that manifests itself is by a gradual introduction of noise into the audio as sections of the signal become too low for the Rx to receive effectively and the signal is lost over time, as the distance increases the signal becomes weaker and weaker and more and more noise is introduced into the system. This is often heard as hiss, or a complete loss of the signal all together and no noise at all as the receiver mutes the channel. Digital RF Whilst Digital radio mics are still susceptible to signal loss the way they deal with this is completely different to analogue radio mics. 1. Digital Modulation schemes Rather than an infinite sine wave being sent between the TX and RX the audio is first quantised by sampling and resolved using bit depth to produce a series of ones and noughts, like any digital data this needs to be sent in packets with both the TX and Rx knowing what is the 1st bit and last bit in each packet ( hence the need for word clock synchronisation). The fact that the RX only needs to differentiate between a 1 and a 0 means that the chance of it miss-interpreting this is much lower so, that for a lower signal ie one close to the lowest receivable level for the receiver the chances of errors is less….that is the theory. However a digital radio mic still needs to send those 1’s and 0’s as an RF signal, and there have been a series of ways to do that. 2. PSK phase shift keying Here the signal changes between +1 and -1 to differentiate between 1 and 0 As the phase changes at the TX the Rx recognises this change and changes the Bit for 1 to 0 or visa versa, this happens very fast. The advantage of this is information can be transferred very fast with a low latency as the RX doesn’t have to wait for very long for the next bit ( as the state is only either 1 or 0) This gives a completely different spectral mask than the one generated by an FM signal. The disadvantage of this is that as the state can only be either 1 or 0 there is a limit as to the amount of information which can be transferred, however this bandwidth is still bigger than the 60db bandwidth of an Analog signal. 3. FSK frequency Shift Keying FSK is similar to PSK in that a change of frequency is the thing that tells the receiver that the state has changed from 1 to 0.. The advantages of FSK are:- Simple process to construct the circuit, Zero amplitude variations, Supports a high data rate. Low probability of error. High SNR (signal to noise ratio). More noise immunity than the ASK Error-free reception can be possible with FSK, Useful in high-frequency radio transmissions Preferable in high-frequency communications. The disadvantages are the amounta of bandwidth it requires compared to ASK or PSK. 4. ASK Amplitude shift keying This relies on the presense or absence of the transmitted frequency. The Advantages of ASK are:- It offers high bandwidth efficiency.It has simple receiver design. ASK modulation can be used to transmit digital data over optical fiber. ASK modulation and ASK demodulation processes are comparatively inexpensive. The odvious disadvantage is that the signal is susepible to noise which increases the error rates considerably. These schemes can be combined and the current modulation scheme for zaxcom radio mics is QAM or 4-QAM which uses a constellation of 4 points in both phase and amplitude to produce data rates much higher than 2 bit PSK, but can be seen as a variation of PSK. The increase in data rate means that the bandwidth of the same carrier wave can carry much greater information so for a single 200khz channel a bandwidth of 120Db can easily be accommodated. However you don’t need to stop there you can also have 16 QAM, 64-QAM etc each with a greater increase in information bandwidth. With Spectrum becoming increasingly sparce broadcasters have seen the inherent advantages of QAM as a modulation scheme, as carrier waves size can be decreased 200 to 125Khz and even 48Khz in ZHD 48 information transferred can increase. However there is a trade off, with increasing points on the constellation diagram, there is an increase in the error rates associated with the transfer of information, and hence latency, the higher the no of points the greater the chance of interference and the higher the latency, hence XR modulation has a latency of 3.5ms and the ZHD48 has a latency of 18ms. The Rx has to wait for the packet of information to arrive, along with any errors which might take place. 5. Intermodulation, RF propagation and signal degradation in the digital domain. As the digital modulation scheme is not reliant on a sine wave the chances of intermodulation are much lower, as harmonics are not present as the carrier wave is not constant. However this does not mean the digital signal is not free of interference, as discussed earlier increased bandwidth is at the cost of error rates, whilst the above diagram is for digital TV the principle is the same for digital radiomics using the same modulation scheme. The wings on the side represent the error rates for transmission across the channel and as such the closer the channel spacing the greater the chance of error rates from the adjacent channel. However what this does mean is that digital channels can be placed next to each other (adhering to the manufacturers channel spacing recommendations ) without the chance of intermodulation between them, hence you can fit 15 channels within a 6Mhz Tv channel (8Mhz for Europe) using XR… 15 x 400KHz = 6MHz, but 30 for ZHD96… 30 x 200KHz= 6MHz. Although with the digital modulation scheme the lower limit for signal reception at the Rx is lower than analog the signal degradation is quite different. Whilst Analog signals degrade in real time as the TX goes away from the RX producing ever greater levels of noise the digital RX will maintain a signal up to the point that the error rates become so great that the audio RF cannot be accurately decoded, and then “drop the signal”. This can be quite unnerving if you are used to some warning with Analog radios as this can come when signal strength still looks quite good and is not something that can be accommodated “in the mix”. This is one of the reasons that range for digital radio mics is traditionally less that for Analog systems, whilst multipath and doppler interference is the main contributing factor to SNR degradation in Analog systems, the need for low error rates and low latency ( in line with their Analog predecessors) in the digital domain requires a level of RF signal ( and hence error rates) that preclude data packet loss in any great amount. Manufacturers can try to mitigate this by adding error correction, but if this is too great audio Artifacts begin to appear and the audio becomes unusable. There has been talk of implementation of OFDM Orthogonal frequency-division multiplexing, which sends parallel date across a series of sub carrier frequencies, however with spectrum becoming less and less available, bandwidth will always be the issue.
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