Exactly. All the film makers would need it. There are few things to consider i guess.

Currently sound man takes the sound on the set in most of the professional productions. And has its own equipments. If there will be remote controller only for the sound man for just sound operations, then he does need to go to the camera and block the cameraman. So there may be an interface just for sound man on the remote controller.

There should be at least 2 input ports to cover most of the set requirements. At the end, sound man would need an external recorder for few cases. So sound man probably choose to buy external recorder.

For most of the documentaries, and some of the low budged projects 1 or 2 low noise XLR on camera interfaces will be enough. But not for all the cases.

I worked as Set Recordist on 10+ productions and tinker with modular synthesizer electronics. Worked with a lot of gear reaching from Aeta 4Minx to various Sounddevices gear. Constructing suitable Preamps, that match those of a field recorder should be possible. Building good 24bit accuracy ADCs can be more challanging though. To achieve high audio quality, some preconditions regarding the rest of the camera have to be fullfilled:

• there has to be a bus capable of accepting 2 to 6 uncompressed 24bit audio streams at 96kHz from the ADCs. The number of those channels has to be discussed. Experience showes that 2 is the minimum. More than two are useful when in addition to the Mic mounted to the camera, radio transmitted laveliers are used. So 4 channels could be very useful already. 6 Channels would be future proof.
• there has to be a very clean symmetric voltage source of ±12 to ±15 Volts for the analog part and a even cleaner voltage source for the ADCs if not available it needs to be created somehow (chargepumps, buffered virtual ground circuits, LDOs, ...)
• 48 Volts for the Phantom power are needed
• there has to be a dedicated analog ground which only touches the digital ground at the PSU. This is important to decouple preamps and ADCs in order to reach the full 24bit accuracy of the ADCs.

This are the preconditions to get good audio in a preamp module going.

For the ADC one could base the designs on NWAGuy's ODAC which was released under CC-BY-ND (we would need to negotiate with him about the ND part). This would output two channels over the USB bus. I am not familiar with the other electronics, but maybe one could save a lot time to fight with ADCs here.

two details:
regarding 48 Volts - they should be switchable for every of the ( preferably 6 ) channels separately.
The ODAC link : http://nwavguy.blogspot.co.at/2012/04/odac-released.html ( btw: look ate THE REAL NUMBERS there )

Bertl wrote:

I would go to pure digital in that box, maybe input and output if you like to use headphones or similar
then stamp and CRC/ECC the data and send it over a simple serial link to the Beta (e.g. via one of the existing shields)
there are two power rails for each shield, one is the IO voltage and the other can be used for powering electronics on the shields (typically 3.3V)

With a focus on realistic goals I would say:

• Inputs: 2x XLR/6.3mm combi (Mic/Line/Balanced)
• Internal Mics: Some cheap and small cardoids for syncing
• Outputs: Digital bus to the rest of the camera, Headphone output with level controls
• Input Controls: Levels Knobs for each channel, Limiter Switch for each channel, Phantom Power switch for each channel
• Metering: Metering bars, maybe a peak and hold LED to reset on each recording
• Interfacing with Axiom Remote?

Open Topics:

• First I feel the question of the physical case should be adressed. There is a Concept https://lab.apertus.org/T761
• Power supply: as mentioned by Bertl we have 3.3V and we need to derive multiple voltages from it ±10 to 15 for decent preamp capabilites, +48V for phantom power. maybe again 3.3V for the ADC parts
• Preamp Circuit Design
• ADC Circuit Design
• Interfacing with the rest

This combination looks really good: https://www.videodevices.com/products/portable-video-recorders/pix-e5/pix-lr (technically and visually!)

In this video Curtis Judd explains why professionals use expensive hardware: https://youtu.be/hDEp34fkbQ4

some points to consider for the audio interface:

• optional high pass filter before pre-amp
• high quality pre-amps
• big gain range
• analog limiter before ADC

I would like to be able to also use such a device standalone as audio field-recorder and USB-audio interface. (like ZOOM or Sound Devices MixPre)

CEntrance MixerFace R4

http://www.pdbmusic.com/index.php/2018/01/centrance-mixerface-r4-mobile-recording/

Zoom F1
2 channel recorder/interface
https://www.zoom.co.jp/products/field-video-recording/field-recording/zoom-f1-field-recordershotgun-mic