Hisss Dual Audio In Hindi 720p Movie
Julia Dodoo
First off you need a 3.5mm jack to twin XLR cable so you can run the outputs of the sound recordists mixer into the Sony A7s. This also allows the recordist to use the same umbilical cable and breakout they would with any other camera.
As the Sony A7s is a microphone level input you will need to set the XLR outputs of the mixer to microphone level so as not to distort the built in pre-amps in the camera. Feeding it a line level (whether at -10dB or +4dB) signal will result in both a noisy and an overmodulating recording. Most recordists with a professional audio mixer (e.g. Sound Devices) should be able to do this.
Moving on to monitoring, most recordists will monitor the output of the camera to ensure all is recording correctly. This is no different with the Sony A7s, we set the output of the headphone jack to 15 (which is the maximum) and this allows you to monitor the sound coming out of the camera in the usual way.
Mick and myself put this setup to good use on a recent shoot where we had several interviews to film throughout a day. The setup worked flawlessly and allowed to production to use the in-camera sound. This not only saved them time in the edit when compared to syncing dual recordings but also allowed us to do quick playbacks on the day and review recordings too.
This camera has the best noise floor of all the DSLR cameras I have come across. The monitoring is also excellent, the throughput being second to none, meaning what you put in (signal) is equal to what you get out( monitored.)
Overall the project was only a days work, low cost mostly using parts I had on hand. The result works great, with amazing sound and power for such a tiny amplifier. My intended use is to provide portable well distributed sound in a conference room and it will work perfectly. The amplifier, power brick, all 4 speakers, 200ft speaker cable and 50ft extension cord all fit within a 16x10x5 box. The issues with speaker pop and bluetooth sounds can be avoided using the volume knob, turning it down when powering and connecting, turning the source to max, then using the volume knob to adjust.
External input. I'd like to have a line input so that it could be used without bluetooth if possible. It wouldn't be hard given the volume is inserted at a line level, would just need to switch over it's input and AC couple the line.
Auto muting/anti-pop. In thinking through how to deal with some of the issues, I realized I could add an output relay so that the speakers are disconnected before the power goes off, and connected after a delay upon power up. This circuit wouldn't be difficult and would eliminate the pop and connecting tones(if connecting right away).
In testing, I've found it quite handy to have this little amplifier and have been thinking about building a bracket that would hold the amplifier, 2 of the speakers and a battery pack all together. The result would smaller than a shoe box, and yet much more powerful than most on the market. Given that it works fine on 12V, I would probably use a 4S hobby battery pack which could easily have a very long run time given how little power this uses.
I selected banana sockets for my output terminals, and mounted them on one edge, 0.75" center between +/-, and 1.0" between the pairs. This spacing is to allow a dual plug to be used, but also to prevent a dual plug from accidentally bridging between the channels. These sockets have a double-D shape to prevent rotation and a nut. I used a rotary tool to cut these slots, and mounted them. (picture taken at the end)
To make it fit in the case, I mounted the volume knob along another side. I intentionally left space for a switch and line input if I ever wanted to add those in the future. (I wanted to, but don't want to spend the time right now) There is a small plastic frame on this side which used to frame the ethernet jack. I cut a rectangular "washer" to fill over this space using galvanized sheet steel. This was later epoxied onto the plastic to make it easier to get together. Drilling the hole in a strip of thin metal like this required sandwiching it between two pieces of wood and drilling all the way through so that the bit didn't grab and rip the steel.
The case didn't have any screw bosses that I could use so I needed to come up with my own way of holding the board in. On one side, the board could lay on a shelf which was held down by the other case half. This would allow the DC jack on the amplifier to be at the edge of the case so I wouldn't have to relocate it, simply cut an opening.
However the other side was still floating. I cut some hooks out of the plastic from a different router and glued them into the case. This allowed the board to be slid under those hooks and layed down oh the shelf on the other side.
The amplifier has two LEDs on it. The Blue one, D2, flashes 3 times about once per second when a bluetooth connection is made. The Red one, D3, flashes 2 times about once per second when a bluetooth connection is lost(only after it was initially made). After power up and before any connection is made, they will rapidly flash alternately. As they are handy, I wanted to be able to see them on the outside as well. Instead of desoldering them and moving them, I just left them on the board, and paralleled additional LEDs. Note that they are not paralleled across the LED itself, but instead, a new LED and new resistor were paralleled across the original LED+resistor. Looked at another way, I tombstoned a new resistor on the 3.8V rail(where the original LEDs were powered from), then ran a wire from that resistor to the LED, and back to the BTM825 pin.
I chose to use a dual color red/green led I had from stock, connected to both the board blue and red. I also added a green led which was simply powered from the 3.8V rail to ground to act as a power indicator.
The router case already had light pipes to direct the on board LEDs out the front. They were long and curved, and I had cut them off in order to fit the amplifier board in the case. I used a small sanding disk to sand the edge flat and then glued the LEDs down to the surface which worked great.
This is one of the two channels. As described before, the circuit takes the differential output of the BTM835, AC couples it to bias to 6V common mode, and converts it to single ended with some gain through a NE5532 opamp. The output, also biased at 6V feeds into a capacitor before the amplifier. If it is truly using a TPA3116, AC coupling like that is required as the amplifier provides it's own bias of 3V.
The output of the opamp I measured up to about +/-0.75V (measured AC mode to remove bias), so is pretty much line level. This means that non-populated connector could be used as a line out, as long as it is AC coupled first to remove the bias.
But why is the audio biased at 6V you might ask? By doing this, the designer could keep the audio directly in the middle of the opamp range, while only using a single polarity power supply.(that is, the opamp is powered by +12V/Gnd and not say +6V/-6V)
A simple 7812 linear regulator is used to provide the 12V rail for the opamp, but also for generating the 6V bias. The 6V bias is simply created with a voltage divider, so is not suitable for providing significant DC current. It does have substantial bulk decoupling, 220uF, so can provide plenty of power at audio frequencies.
Knowing this circuit, we can understand more about the input voltage. The module is labeled 12V-24V. The 24V max is sensible given the TPA3116. But I would have expected the minimum to be closer to 14V as the rail is used through a (non-LDO) linear to create 12V. In the end, running it at 12(and lower) will work, it will just change the circuit as the opamp will then run around 10, and the audio bias around 5.
I did test this with input voltages as low as 11 to work fine as I might want to battery power the module in the future.(4S lithium ion modules are much more common than 5S or 6S) In testing, current stayed below 100mA until the volume started getting pretty loud. It wasn't until near maximum(too loud for me in the same small room as the speakers) that the current went about 1A, with the highest peak I saw around 4A.(I did not use a scope so didn't catch quick pulses very well) Partly because of the low current draw, I decided not to add any additional input capacitance as the amplifier already has 6*330uF caps across the rail. If running the amplifier near it's max, it might help to add some more, but I would recommend doing some testing first to see what voltage drop you have from your supply and how high the currents peak.
I switched to a stereo/dual audio taper potentiometer as a volume control. I didn't have many audio taper stereo pots in my stock, and initially used a 50k one. This worked, but increased hiss, and was more subject to 60hz hum. I switched to a 10k one which improved the situation, removing noticable hum, and reducing hiss down to a low level. The hiss and hum was mostly when the pot was set away from the ends, so it seems the amplifier doesn't want an input impedance this high. A 1k would have been much better if I had one or was purchasing one.
This is one of two channels shown. Notice that the other end of the pot(CCW rotation) is connected to 6V, and not ground. This is because at this point in the circuit, it has a 6V common mode. If I were to connect it to ground, excess DC current would flow through the potentiometer. This current would come from the 6V rail, which is only weakly generated with a divider so it would have changed the circuit undesirably.
Another option would have been to insert an additional AC coupling capacitor out of the opamp before it goes into the pot to bias it to ground; then the CCW terminal of the pot could have been connected to ground. This may have increased noise however as ground noise would now be included.