Raspberry Pi as an NMEA Multiplexer

So I got an AIS 'transponder'. They call them transponders - it says it on the box. But it isn't actually a transponder. A transponder transmits in response to a probe. AIS 'transponders' are beacons. They transmit even if nothing is probing them.

Anyway, I want the AIS targets displayed on the chart plotter and the chart plotter only has one NMEA input which is already used for depth so I need a multiplexer.

I'm using a Pi3 with USB/Serial adaptors. The plan was to use all of the four USB sockets with one adaptor going to the AIS transponder, one to the chart plotter, one to the DSC VHF radio and one to the depth device.

But one of my USB ports doesn't work. So I'm sharing one device for the depth and the VHF. The input comes from the depth unit and the output goes to the VHF. I don't have anything to get the DSC targets from the radio but that's OK for now.

There are (at least) two choices for software NMEA multiplexors on Linux. The most mature is kplex. The alternative is Signal K. I'm using both but I'm only using kplex in anger.

There are problems.

1. The wifi is configured as a hotspot. It doesn't always start-up properly. I've put a 10s delay in the init.d script for now and that seems to fix it but its a hack.
2. I can't work out how to make the USB/Serial ports always end up on the same device names.

There is lots of advice on the internet about USB naming. The answer is to write some UDEV rules to sort it out. This works OK for the single PL2303 device but I haven't come up with anything for the two indistinguishable Cxxxx devices.

I think I need to make kplex auto-baud and auto detect the type of device from the sentences it outputs.

Raspberry Pi Kodi CEC for Yamaha A/V Receiver

CEC works out of the box on Kodi on the Pi but not if you have an A/V Receiver and a projector rather than a TV.

If can put put right in the settings in Kodi if you have a working remote but a mouse doesn't seem to do it.

Copying the relevant config file from a working system is the easiest way. For me this file gets created as:
 
/home/kodi/.kodi/userdata/peripheral_data/rpi_2708_1001.xml

2708:1001 is the device ID for the pi's CEC adapter.

The contents of my file are:

<settings>
    <setting id="activate_source" value="0" />
    <setting id="button_release_delay_ms" value="0" />
    <setting id="button_repeat_rate_ms" value="0" />
    <setting id="cec_hdmi_port" value="1" />
    <setting id="cec_standby_screensaver" value="0" />
    <setting id="cec_wake_screensaver" value="1" />
    <setting id="connected_device" value="36037" />
    <setting id="device_name" value="Kodi" />
    <setting id="device_type" value="1" />
    <setting id="double_tap_timeout_ms" value="300" />
    <setting id="enabled" value="1" />
    <setting id="pause_playback_on_deactivate" value="1" />
    <setting id="physical_address" value="0" />
    <setting id="port" value="" />
    <setting id="send_inactive_source" value="1" />
    <setting id="standby_devices" value="231" />
    <setting id="standby_devices_advanced" value="" />
    <setting id="standby_pc_on_tv_standby" value="13005" />
    <setting id="standby_tv_on_pc_standby" value="1" />
    <setting id="tv_vendor" value="0" />
    <setting id="use_tv_menu_language" value="1" />
    <setting id="wake_devices" value="231" />
    <setting id="wake_devices_advanced" value="" />
</settings>

MPEG-DASH and HLS with Gstreamer

Gstreamer does a good job with the modern streaming protocols.

The command line client gst123 can be used to listen to BBC AoD and live streams.

BBC AoD streams can be found here: BBC World Service AoD feed (replace worldservice with your channel of choice).

BBC Live streams can be found here: BBC IMDA transports

Here are some examples using gst123:

Play an MPEG-DASH stream listed in imda_transports.xml:

gst123 -q http://a.files.bbci.co.uk/media/live/manifesto/audio/simulcast/dash/nonuk/dash_low/ak/bbc_radio_three.mpd

Play an HLS stream listed in imda_transports.xml:

 gst123 -q http://a.files.bbci.co.uk/media/live/manifesto/audio/simulcast/hls/uk/sbr_vlow/llnw/bbc_radio_three.m3u8

Play an on-demand DASH stream from Radio 4 (you will need to look in the xml file for a current programme):

 gst123 -q http://open.live.bbc.co.uk/mediaselector/5/redir/version/2.0/mediaset/audio-syndication-dash/proto/http/vpid/b084d7wf

It is really easy to roll your own Gstreamer based player using python. There are some examples here.

I rolled this one by adding command line processing to basic-tutorial-1:

#!/usr/bin/env python3
# -*- coding:utf-8 -*-
import sys
import gi
gi.require_version('Gst', '1.0')
from gi.repository import Gst
Gst.init(None)

# Build the pipeline
pipeline = Gst.parse_launch("playbin uri=%s" % sys.argv[1
# Start playing
pipeline.set_state(Gst.State.PLAYING)

# Wait until error or EOS
bus = pipeline.get_bus()
msg = bus.timed_pop_filtered(
    Gst.CLOCK_TIME_NONE, Gst.MessageType.ERROR | Gst.MessageType.EOS)
# Free resourcespipeline.set_state(Gst.State.NULL)

Then you can use the same example streams:

./gst.py http://a.files.bbci.co.uk/media/live/manifesto/audio/simulcast/dash/nonuk/dash_low/ak/bbc_radio_three.mpd

./gst.py http://a.files.bbci.co.uk/media/live/manifesto/audio/simulcast/hls/uk/sbr_vlow/llnw/bbc_radio_three.m3u8

It only takes a little more effort to make something useful. The following python3 script takes the name of a programme and the url of an AoD feed on the command line and plays the latest episode of that programme:

# Free #!/usr/bin/env python3
# -*- coding:utf-8 -*-
import sys
import gi
gi.require_version('Gst', '1.0')
from gi.repository import Gst
from xml.etree import ElementTree
from datetime import datetime
import arrow
from urllib.request import urlopen

Gst.init(None)

# Get a url
title = sys.argv[1]
feed = sys.argv[2]
now = arrow.utcnow()
wanted = now.replace(hours=-1)
with urlopen(feed) as f:
    tree = ElementTree.parse(f)
for node in tree.iter('entry'):
    for entry in node.iter('parent'):
        if entry.text == title:
            for field in node.iter():
                if field.tag == 'link' and field.attrib.get('transferformat') == 'dash':
                    thisurl = field.text
                if field.tag == 'availability':
                    start = arrow.get(field.attrib.get("start"))
                    end = arrow.get(field.attrib.get("end"))
                    if (start <= now) and (now <= end) and (start > wanted):
                        wanted = start
                        url = thisurl
# Build the pipeline
pipeline = Gst.parse_launch("playbin uri=%s" % url)

# Start playing
pipeline.set_state(Gst.State.PLAYING)

# Wait until error or EOS
bus = pipeline.get_bus()
msg = bus.timed_pop_filtered(
    Gst.CLOCK_TIME_NONE, Gst.MessageType.ERROR | Gst.MessageType.EOS)

# Free resources
pipeline.set_state(Gst.State.NULL)

So you can do this:

./bbcaod.py   "BBC News" http://www.bbc.co.uk/radio/aod/availability/worldservice.xml

Google Maps Location Sillyness

Google Maps has a funny idea of where things are.

If I view the 'my contributions' page it looks something like this

See that marker in the middle of the North Sea? And the photo on the left which says North Sea?

Now look at this image of Google Maps zoomed in to near Harwich

That's the same picture.

When I look at this on Panoramio it says the location is Harwich.

In fact it says "Photo taken in 13 Lower Marine Parade, Harwich CO12 3SS, UK

51° 54' 50.75" N  1° 18' 15.42" E"

Which is clearly not true but better than "North Sea".

I can't find any way of adding names of seas, bays or channels or of changing where this is, or how come Google Maps says its in the north sea but Panoramio says its in Harwich.

OpenCPN is in the Play Store

Which is great news but it runs very poorly on my Galaxy Tab2 7". I think it needs to be a lot more optimised to work properly.

I can use my Antares charts on it and in theory OpenSeaMap but those folks are not good at making their work easily accessible. I'll find out how best to do it and post that here.

OpenCPN has great built-in NMEA multiplexing so that part is not a problem. I can get my dummy AIS target displayed just fine.

Of course OpenCPN has access to proper charts via VisitMyHarbour. On Windows.

Of course VisitMyHarbour provide proper charts for Android. On Marine Navigator.

Marine Navigator doesn't do AIS or any NMEA overlays.

Serial to Wifi

It looks like the ESP8266 may be the way to do this. I'm not sure if it can do a full TCP relay but I think it can. It needs NMEA 0183 level shifting but apart from that it seems like the perfect low cost way to get away from NMEA wiring.

AIS Engine and Android

A friend asked last night how to connect his Nasa AIS Engine to his Google Nexus tablet so he could see AIS plots on his charts.

He uses Marine Navigator and has the tablet plugged in to the boat power all the time to keep it charged.

I did a quick search and as I expected it is complicated...

Marine Navigator doesn't support AIS - at least it doesn't mention it  on the Google Play page.

MemoryMap does. He would have to buy his charts all over again though. When are we going to get chart portability between apps?

MemoryMap does - NMEA input via wifi or Bluetooth including DSC and AIS.

So the obvious thing to do would be to connect an NMEA to USB converter like the Actisense one to the tablet.

Not so simple. Nexus tablets are rumoured not to support OTG devices when charging.

Not so simple. Android doesn't support serial devices.

I did find one piece of good news. The NMEA output on the Nasa AIS engine is RS232 compatible. So we don't need an expensive NMEA to USB converter. A cheap RS232 to USB converter will do.

Other good news is that libusb works on Android and there is open source software to connect to a libusb connected serial port adapter if it has either an FTDI chip (most do) or a pl2303 chip (mine does).

So there is a way to do it if the charging problem isn't real:

1) Connect the AIS Engine to an RS232 to USB adapter.
2) Connect the RS232 to USB adapter to a Nexus compatible USB OTG adapter.
3) write a little app that uses the open source libusb and serial port drivers and acts as a tcp/ip server for NMEA messages
4) install Memory-Map

If the charging problem is real, or we want to be able to walk around the boat with the tablet, then we can put the app on a Raspberry Pi instead and MemoryMap can connect over wireless.

So this is my bill of materials for the Pi solution.

Hardware:
1 x raspberry Pi 1 Model B (for example)
1 x SD Card
1 x 5V 3A radio control model BEC (DC-DC converter)
1 x RS232/USB Adapter
1 x USB Wifi Module
1 x Mini USB cable
1 x box to put it in

Software
raspian
kplex

I'll get this working and feed it some canned AIS messages and see if I can overlay them on memory map.

It turns out the AIS features in MemoryMap are "pro" features and cost over $100. That's not what we want.

The demo I tried didn't let me use the internal GPS and an external (tcp) AIS receiver. I don't know if this is fundamental but it breaks the model for me.

I wrote an add-on for gpsfeed+ which makes it generate an AIS target near the track in simulation mode. This is just the kind of test software I need for making everything BUT the RF ais decoder work in my NMEA based projects. That is really useful.