With the Mark5A recording system, the maximum bit rate that can be normally be recorded is 1024 Mbps on a Mark IV system and 512 Mbps on a VLBA system. These rates are recorded on a single module, unlike in the tape era when 2 drives or 2 heads were required.
SCHED can make schedules for the 512 Mbps and 1Gbps modes. See the examples eg512.key for a VLBA only case and eg2head.key for a PCFS (MarkIV) case. For the user, 512 Mbps modes are not much different from other modes. The VLBA telescope schedules indicate use of the wide band mode simply through the specification of track numbers above 64. Note that the two examples do either only VLBA or only Mark IV, but it is ok to mix them.
New digital backends and a new recording system are starting to be tested as of this January 2011 writing. These will increase the available bit rate to significantly higher values. The RDBE/Mark5C system under development at Haystack and NRAO will initially record 2048 Mbps which will increase to 4096 Mbps soon afterward, assuming funds can be found. The DBBC system, also using the Mark5C recording, system is being deployed on a similar time scale and should have similar bandwidths.
The RDBE (Roach Digital Backend, where ROACH is the core board containing a large FPGA) is a module that takes in 2 analog IF signals, applies an anti-alias filter that passes 512 to 1024 MHz, sets the power levels, samples the signals at a 1024 MHz sample rate (8 bit samples at this stage), digitally filters the data to the final basebands, resamples the data to 2 bits, and formats it for recording. It takes the place of the IF distributers, baseband converters, samplers, and formatter (including pulse cal detectors) in the old VLBA system.
Control of the RDBE and Mark5C recorders is handled by a new VLBA control system running on a standard Linux computer. The new system is based on the EVLA Executor. Schedule information it given to this computer by way of the VEX file, which is converted by operations to a Python script that is read by the Executor. All new hardware installed at the VLBA for the next few years will use this control system and, probably slowly, the old hardware will be switched over to it. In the meantime, both crd files and VEX files will be needed to control the VLBA sites.
Note that what are called basebands in the new system, for backward compatibility, are known as subbands in the terminology used on the somewhat similar EVLA system and the two terms can be used interchangeably for the VLBI systems. However, for the EVLA, the word ``baseband'' is used for the final analog signal sent to the samplers and is equivalent to the ``IF'' used here. We have opted not to use that terminology because of the massive confusion it might create by swapping terms used for different signals compared to previous practice.
The FPGA in the RDBE supports multiple personalities that can be swapped as needed. The first developed uses a polyphase filter to break the 512 MHz band into 16 basebands of 32 MHz each, all lower sideband. This personality is selected using the DBE parameter set to RDBE_PFB in the setup file. Actually, there are 15 bands of 32 MHz and one 16 MHz half band at each end because the end filters are centered on the band edges. In the initial implementation, every other one of the full 32 MHz bands was taken from each of the 2 IFs, creating 16 basebands total for a total bit rate of 2 Gbps. An enhancement expected shortly (early 2011) is to allow any 16 of the 30 full 32 MHz bands to be selected. This personality can only provide 32 MHz basebands at fixed frequencies for a total of 2 Gbps. No variations on those parameters are supported. Note that eventually there will be 2 RDBE modules at each station to support 4 Gbps.
The second personality that will be available is the DDC (Digital Down Converter). It is selected using the DBE parameter set to RDBE_DDC in the setup file. This personality will provide 8 filters per RDBE (typically 4 on each IF) that can provide arbitrary offset frequencies and can provide any bandwidth at the factor of 2 steps between 0.125 and 64 MHz. There is a complication forced by the use of a polyphase filter first step of filtration to get the clock rate down to values the FPGA can support. Such filters do not have flexible frequency ranges. This one splits the band into 3 segments, 512 to 640 MHz, 640 to 896 MHz, and 896 to 1024 MHz. Each baseband must be confined to one of those ranges. The ``crossover'' frequencies at the filter edges have a range of something like 4-10 MHz (to be determined) that is not really usable. SCHED will issue a warning if an attempt is made to have a baseband cross one of these boundaries.
The frequencies for the band edges in the DDC personality can be set to any multiple of 1 Hz in principle. But, for now, the set points are being limited to 10 kHz intervals as with the BBCs. This is because throughout SCHED these numbers are stored in single precision variables that cannot support more precision and in the various output files, including those used to control the telescope hardware, not enough digits are printed. At some point, if there is demand, these issues will be addressed.
The overall LO/IF/RDBE system on the VLBA will have some significant tuning flexibility issues. The RDBE is an addon to the older system where the baseband converters, which could take only a small portion of the 500 MHz IF, provided the necessary flexibility. The LO/IF system that creates those IFs is based on synthesizers that have set points at N*500+-100 MHz. Now, with the ability to take all of an IF, that restricted tuning ability will become an issue, especially in conjunction with the lack of tuning ability for the PFB personality and the crossover points for the DDC. Essentially all frequencies can be reached using more than one LO setting, so no cases have been identified where a particular spectral line cannot be observed. But full tuning flexibility that might be desired is not there. Eventually we hope to upgrade the front end synthesizers to designs with more tuning options, but that is not yet funded.
Note that, for the initial /schedb implementation in place (Jan 2011, version 9.4) the code to provide default channel frequencies based on the band has not yet been written. It is necessary to give the frequencies in the setup file. See the examples. The defaulting capability will be added eventually.
During testing of the RDBE/MARK5C system, it is useful to have a parallel Mark5A recording. That can be requested with parameter switch /htmlrefDOMKAMP:DOMKA.
The DBBC being developed for the EVN is also a system that samples at 1024 MHz and digitally filters the signals to the desired bandwidths. But it has a different design where, like with the old BBCs, the frequency can be set flexibly anywhere in the IF band without concern about crossover frequencies etc. The DBBC design has a module for each output baseband, so they are more directly comparable to BBCs. More information is needed abou the system to flesh out this discussion.
There is a varient on the RDBE being developed at Haystack for mm VLBI that has 4 input IFs and does not attempt any filtration. It simply samples and formats the data and sends it to the recorders. It can put out 8 Gbps. This device is not yet supported by SCHED.
When the DAR is the RDBE, the output channels and all the input channel information given to SCHED are written to the VEX file. But the crd files that control the old VLBA hardware also has to be told something. SCHED does not have a separate set of variables for all those configuration parameters, so it just does something reasonable. It sets the number of channels to the maximum of the number requested and 8. It sets the frequencies and sidebands to match the RDBE requests. It sets the sample rate to the maximum of that requested and 32 Ms/s. It sets the channel bandwidth to the lesser of the request and 16 MHz. It only writes the first 4 pcal extraction requests (avoiding going into channel numbers that are too high).