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borjam

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About borjam

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  • Location
    Bilbao/Spain
  • Interested in Sound for Picture
    Yes
  • About
    Bilbaina Jazz Club

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  1. borjam

    Any Drummers out there

    Gymnastics before a concert
  2. borjam

    Sound Devices Mix Pre-3 and Mix Pre-6

    I saw the inside photos of the MIxPres on the FCC applications and i noticed a Zynq 7000 SoC. I guess there's a memory limitation (note, I didn´t check everything thoroughly). Interestingly these SoCs have been behind the launch of some surprisingly capable (for the price) oscilloscopes. https://www.xilinx.com/support/documentation/selection-guides/zynq-7000-product-selection-guide.pdf On a MixPre 3, for instance, 5 seconds at 96 KHz need 1.4 MB. For 5 tracks (3 ISOs and a stereo mix) that makes roughly 7 MB. These SoCs don't have a lot of internal memory. You can add if of course (several variants of DDR are supported) but with a power consumption penalty. So I'm not sure wether it was a deliberate decision not to include an "obvious" feature in the MixPres (as I think happened with the polarity reverse for the MixPre 3) or it has demanded an extra effort to squeeze everything together. I am sure there's been a "stormy" chorus of voices asking them to add pre record especially in the nature field recording camp. I remember I read a criticism from a very influential figure from Cornell University. Whatever, it's a good thing. I don't think prerecord will be a power consumption problem, I checked which settings had an effect on power consumption using a laboratory power supply and even when not recording the audio engine is working. When I tested it I was wondering wether an aggressive power management really powered off preamplifiers for channels not in use, etc.
  3. borjam

    Sony F5 - F55 - FS7 RF Spray problems.

    It depends on the particular interference problem. But I guess that part of that RF spray will be in the same frequency you are using for your microphones, which means the filters won't help. On the other hand I doubt high end wireless microphones will have trouble even with interfering signals close to the intended working frequency. One of the main differences between expensive and el-cheapo RF circuitry is filtering. And I imagine that the spray won't be so strong to cause intermodulation. How did the RF Explorer measurements look?
  4. borjam

    MixPre-10T Rec/Stop button (failed)

    Maybe it's enough to add debugging code to your device. I imagine (but it's just an educated guess) that the software you are using is supporting just the bare minimum subset of the HID/keyboard commands and the MixPre is using the whole set of commands a standard keyboard should support. So, probably either it's not answering some important message or it's sending the wrong answer. I would check the program and add a handler for unprocessed USB requests just in case. That should give a good hint of what's going on.
  5. borjam

    MixPre-10T Rec/Stop button (failed)

    Pity they don't disclose it. If I have time I can try hooking a keyboard to my MixPre and snoop with the oscilloscope. Do your devices implement a complete keyboard emulation? Perusing the HID specifications, keyboards are more complex than they seem! Why don't you enable debugging in your program and check which requests is the MixPre sending to your unit? I guess it's failing to answer to some keyboard/HID description or status packets, probably it would be simple to forge a proper answer. I'll see if I can hook a DSO to a real keyboard connected to my MixPre and decode the USB exchange with Sigrok.
  6. borjam

    DIY Dipole antenna!

    Capacitive hat?
  7. borjam

    Recording music performances on set

    Will they use music stands and scores? Those will help. And even in jazz improvisations it's not unusual to have some scores around.
  8. borjam

    DIY Dipole antenna!

    Oh, ok, sorry, I stand corrected! I didn't search for it and I assumed some esoteric cable. I would recommend you to explore the option of colinear antennas. In very simple terms a colinear is a vertical stack of vertical antennas. It's omnidirectional in the horizontal plane but with a lower radiation angle (ie, it's directional in the vertical plane). That means you have some gain and, as a plus, it will help reject multiple reflections coming from the ceiling and floor. https://en.wikipedia.org/wiki/Collinear_antenna_array At those frequencies antennas are not large and this is less cumbersome than a log-periodic array.
  9. borjam

    DIY Dipole antenna!

    You don't need fancy oxygen free copper or anything of the sort. Also, remember that at high frequencies current density is much higher on the surface of the connector (it's called skin effect) so you can use a thin hollow tube instead of solid wire. It doesn't need to be copper even, aluminum can do as long as you adjust the length properly. That's where a VNA is so handy It will help you know wether you did a good job. It can drive you nuts as well, because you will notice that stuff getting close to the antenna can alter its performance significantly. I guess manufacturers of wireless microphone transmitters consider the effect of a human body almost touching the antenna. Dipoles are essentially single frequency antennas given that the intended impedance is constant. And their performance and impedance is more or less the same. Around 75 ohm for the half wavelength dipole. There are also broadband and multi band antennas. Broadband doesn't necessarily mean much less efficient, the three parameters are size, bandwidth and efficiency. If the antenna is not small (always related to wavelength) you don't need to trade efficiency for bandwidth. The log periodic "axe" antennas are a good example of that. Yes. If everything is properly made (that's why I suggested SV1AFN's PCBs) the path from the electronics to the connector should be close to ideal. At high frequencies PCB layout and traces are so important, it's usually said that you need Maxwell's laws to design the circuit rather than using simply the good old Kirchoff's laws. It will really make a difference if you are building your own antennas. You will find out that cutting them for the right length is tricky at high frequencies. That said: are you intending to test transmission antennas? I am not sure about the legal status of antenna experimentation with wireless microphones. I am an amateur radio operator and we have "legal superpowers" regarding experimentation. But the radio equipment used outside of the amateur bands is subject to approval by the relevant authorities and often both transmitter and antenna are approved in combination. You can't generally replace transmission antennas at will. You might get in trouble if a poorly matched transmission antenna caused, for example, distortion, and you ended up transmitting spurious distortion products interfering other bands. You might have a look at these two books if you want to delve deeper. Beware, they can contain much more detail than you want, just skip the stuff you don't need/want. The ARRL Antenna book has lots of practical information. It's updated frequently with new designs. And for a not so gentle introduction to antenna theory skipping most of the math, Joseph Carr's "Practical Antenna Handbook" is very good. But sometimes not for the faint of heart https://www.amazon.com/Practical-Antenna-Handbook-Joseph-Carr/dp/0071639586
  10. borjam

    Sound Devices Mix Pre-3 and Mix Pre-6

    I just found a bug (MixPre 3, fw version 2.21). I've sent it to SD, but sharing here because when it hit me it looked like I had a defective preamp. Workaround included. When linking channels 1 and 2 in stereo, after unlinking them while keeping channel 2 (which, when linked, is a balance control) centered, channel 2 seemed noisier. Turns out some gain offset is applied to it, and channel 2 has a higher gain than channel 1. This at least happens with the gains in Basic mode (the rest of the options are in Advanced mode). The workaround is simple: after unlinking, if this condition occurs, set the channel 2 input to Line and back to Mic. It will reset the preamp properly.
  11. borjam

    DIY Dipole antenna!

    Yes. Simple antennas resonate well at a given frequency, more elaborate designs can be multi band. Others can be narrow band. The HF loop resonating on 7 MHz for instance has a bandwidth of just 2 or 3 KHz. In general it’s say that there are three properties of which you can choose two, compromising the one left. Bandwidth, efficiency, size. The loop is tiny (a diameter of 1 metre for a wavelength of 40 meters). Designing antennas is still a bit of a black art. There are mathematical modelling tools like NEC but at the end you need to fine tune the prototype and, in the first place, test it. And modelling tools such as NEC don’t always produce accurate results. Loops are one of the corner cases for example. Their performance according to the models violates the first principle of Thermodynamics In principle a good antenna for transmission will work for reception. Antenna matching is more critical for transmission because a badly matched antenna can affect the performance of a transmitter and, in some cases, damage it or at least make it distort. I’ll dig a couple of books in case you want to read some. Actually you will find that some of your acoustic intuition applies to antenna resonance, just replace the speed of sound by the speed of light
  12. borjam

    DIY Dipole antenna!

    Do you mean, for example, products from Minicircuits? https://ww2.minicircuits.com/WebStore/Transformers.html For the proper way to mount them on a PCB, this guy can be helpful. I am not affiliated to him but I'm a happy customer. https://www.sv1afn.com I guess he can be commissioned for some custom work. I would give him a call. For example, if you wish to experiment with SMD components these PCBs can be really useful: https://www.sv1afn.com/rf-experimenter-s-pcb-panel.html They have a proper coplanar line to keep the 50 ohm impedance from the SMA connector to your components.
  13. borjam

    DIY Dipole antenna!

    Vector Network Analyzer. It's an instrument that allows you to perform several measurements for a circuit and/or element. It has some practical purposes when dealing with antennas, filters and transmission lines. You can check the actual impedance of an element, detect damage in coax lines, check the resonance frequency of an antenna, measure the attenuation of a transmission line or filter, check its band pass, etc. It can help detect damaged connectors and some of the usual RF "witches". It's a very specialized (and expensive!) instrument. But lately some cheap units have been released for the radio amateur market. If a "real" VNA can be in the tens of thousands of dollars, the "toy" but useful ones are in the less than $500 mark and they can be real time savers. In the radio amateur market they are sometimes sold as "antenna analyzers". I own one of the cheap ones (miniVNA Tiny) and despite the ridiculously low price it's surprisingly useful. The attached PDFs show what one can expect. One of them is a test of an HF antenna (a loop) tuned to the 7 MHz band and the other one a test of a sample UMTS/LTE/WiFi antenna we ordered from a Chinese manufacturer. The third one shows a misalignment in the front end filtering of a broadband receiver (should be centered on 140 MHz, the tuning frequency, while it was off by 9 MHz) and the fourth one shows a defective filter which was in theory a band-reject for the VHF broadcast band. Turns out it had been assembled with the wrong capacitors. Not bad for a $500 instrument I would say. VNA_150811_101602.pdf VNA_170319_232236.pdf VNA_170924_124857.pdf VNA_170320_194207.pdf
  14. borjam

    DIY Dipole antenna!

    If you want to go this route, a simple VNA (real RF guys look elsewhere!) can help. I have a miniVNA Tiny and for the price (and with its huge limitations) it's surprisingly helpful. For instance it helped me choose some UMTS/LTE/WiFi antennas and the results were consistent with the measurements I made at the University with a real über expensive Agilent VNA.
  15. Beware, a couple of cm make a difference in UHF frequencies. That said, the effect is much more dramatic in a transmitter than in a receiver. But audiofp's advice is really good: in a marginal situation a silly mistake can tip the balance to the bad side
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