Update docs and such for 1st Gen and big 4th Gen support

docs/iface-1st-gen.md

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+# ALSA Scarlett Control Panel
+
+## Scarlett 1st Gen Interfaces
+
+This document describes how to use the ALSA Scarlett Control Panel
+with the Scarlett 1st Gen interfaces:
+
+- Scarlett 1st Gen 6i6, 8i6, 18i6, 18i8, 18i20
+
+Note: The 1st Gen Scarlett Solo, 2i2, and 2i4 have all their controls
+accessible from the front panel of the device, and there are no
+proprietary software controls, so they do not require this control
+panel software.
+
+## Important Driver Limitations
+
+The 1st Gen Scarlett devices have some important limitations in the
+ALSA driver implementation that you should be aware of:
+
+1. **Initial State Detection**: The driver cannot read the current
+   state of hardware controls (this appears to be a limitation of the
+   device firmware). When alsa-scarlett-gui starts, what you see will
+   not reflect the actual state of your device unless the controls
+   have previously been set since startup.
+
+2. **State Update Issues**: The driver only updates the hardware state
+   when it thinks a setting needs to be changed. If the driver
+   incorrectly believes a control is already in the desired state, it
+   won't actually update the control.
+
+### Recommended Workaround
+
+To ensure your settings are properly applied:
+
+1. Apply a "zero" configuration that sets all controls to values that
+   are *not* what you desire.
+2. Then apply your desired configuration
+
+This two-step process helps ensure that the driver actually sends all
+commands to the hardware. You may want to create a script using
+`alsactl` for this purpose.
+
+## Main Window
+
+The main window is divided into three sections:
+
+- Global Controls
+- Analogue Input Controls
+- Analogue Output Controls
+
+The particular controls available depend on the interface model.
+
+Note that the View menu option lets you open two other windows which
+contain additional controls, described in the following sections:
+- [Routing](#routing)
+- [Mixer](#mixer)
+
+The Levels and Startup windows that are available for later-generation
+interfaces are not available for 1st Gen interfaces due to driver limitations.
+
+### Global Controls
+
+Global controls relate to the operation of the interface as a whole.
+
+#### Clock Source
+
+Clock Source selects where the interface receives its digital clock
+from. If you aren't using S/PDIF or ADAT inputs, set this to Internal.
+
+#### Sync Status
+
+Sync Status indicates if the interface is locked to a valid digital
+clock. If you aren't using S/PDIF or ADAT inputs and the status is
+Unlocked, change the Clock Source to Internal.
+
+### Analogue Input Controls
+
+#### Inst
+
+The Inst buttons are used to select between Mic/Line and Instrument
+level/impedance. When plugging in microphones or line-level equipment
+(such as a synthesizer, external preamp, or effects processor) to the
+input, set it to “Line”. The “Inst” setting is for instruments with
+pickups such as guitars.
+
+#### Pad
+
+Enabling Pad engages a 10dB attenuator in the channel, giving you more
+headroom for very hot signals.
+
+#### Gain
+
+The Gain switch selects Low or High gain for the input channel.
+
+### Analogue Output Controls
+
+The analogue output controls let you set the output volume (gain) on
+the analogue line outputs.
+
+Click and drag up/down on the volume dial to change the volume, use
+your arrow keys, Home/End/PgUp/PgDn keys, or use your mouse scroll
+wheel to adjust. You can also double-click on it to quickly toggle the
+volume between off and 0dB.
+
+## Routing
+
+The routing window allows complete control of signal routing between
+the hardware inputs/outputs, internal mixer, and PCM (USB)
+inputs/outputs.
+
+![Routing Window](../img/scarlett-1st-gen-6i6-routing.png)
+
+To manage the routing connections:
+
+- Click and drag from a source to a sink or a sink to a source to
+  connect them. Audio from the source will then be sent to that sink.
+
+- Click on a source or a sink to clear the links connected to that
+  source/sink.
+
+Note that a sink can only be connected to one source, but one source
+can be connected to many sinks. If you want a sink to receive input
+from more than one source, use the mixer inputs and outputs:
+
+- Connect the sources that you want to mix together to mixer inputs
+- Connect mixer outputs to the sinks that you want to receive the
+  mixed audio
+- Use the Mixer window to set the amount of each mixer input that is
+  sent to each mixer output
+
+The Presets menu can be used to clear all connections, or to set up
+common configurations:
+
+- The "Direct" preset sets up the usual configuration using the
+  interface as a regular audio interface by connecting:
+
+  - all Hardware Inputs to PCM Inputs
+  - all PCM Outputs to Hardware Outputs
+
+- The "Preamp" preset connects all Hardware Inputs to Hardware
+  Outputs.
+
+- The "Stereo Out" preset connects PCM 1 and 2 Outputs to pairs of
+  Hardware Outputs.
+
+## Mixer
+
+If you use the Routing window to connect Sources to Mixer Inputs and
+Mixer Outputs to Sinks, then you can use the Mixer window to set the
+amount of each Mixer Input that is sent to each Mixer Output using a
+matrix of controls.
+
+Click and drag up/down on the gain controls to adjust, or use your
+mouse scroll wheel. You can also double-click on the control to
+quickly toggle between off and 0dB.