The network interface (and MAP65)
Linrad can multicast data from several points in its pipeline, and can take data from the network as input. This lets several operators share one receiver, distributes the processing across machines, and feeds external decoders such as MAP65. From z_NETWORK.txt.
Formats and tap points#
Any of these formats can be sent simultaneously; each goes to the base port plus a format-dependent offset:
| Format | Offset | Description |
|---|---|---|
| RAW16 / RAW18 / RAW24 | 0 / 1 / 2 | raw A/D data (16/18/24-bit) |
| FFT1 | 3 | first-FFT transforms (float) — the wideband spectrum |
| TIMF2 | 4 | noise-blanker output (16/32-bit int/float) |
| FFT2 | 5 | second-FFT transforms (16/32-bit int/float) |
| BASEBAND | 6 | baseband I/Q (16-bit) |
Multicasting uses groups 239.255.0.0–239.255.0.15, so several receivers can send in different groups at once; the port is set between 50000 and 65000 in steps of 10. Each packet carries a NET_RX_STRUCT header (passband center/direction, timestamp, block number, pointer) ahead of the payload.
In the source
Send functions per format: lir_send_raw16/18/24, lir_send_fft1, lir_send_timf2, lir_send_fft2, lir_send_baseb, lir_send_basebraw — network.c:362; driver do_network_send() / network_send() — rxin.c:669 (thread THREAD_NETWORK_SEND). Output-format flags NET_RXOUT_* and input flags NET_RXIN_* in ui.network_flag — globdef.h:237. Packet header NET_RX_STRUCT and the per-format net_rxdata_* buffers — globdef.h:1284; payload size NET_MULTICAST_PAYLOAD (globdef.h:1283).
Network as input#
RAW or FFT1 transforms received from the network can be used as Linrad's input, so several operators listen to the same receiver/antenna, or one machine does the early processing and sends a later stage on (same base port, different group) for another machine to continue.
In the source
Network-input threads THREAD_RX_RAW_NETINPUT and THREAD_RX_FFT1_NETINPUT (thrdef.h:48); receive setup and the par_netrec_ip mechanism in network.c (par_netrec_filename, network.c:121). Input format is selected by the NET_RXIN_* bits of ui.network_flag.
MAP65 and arbitrary destinations#
For a decoder such as MAP65 (wideband EME) Linrad can multicast to any IP address rather than only the 239.255.0.x groups: put the destination in a file par_netsend_ip and set −1 for the last number of the send address in the network setup (then press W in the main menu). The receive address can likewise be read from par_netrec_ip. MAP65 receives a Linrad wideband stream over this multicast.
In the source
par_netsend_ip / par_netrec_ip handling — network.c:120; the −1 last-octet convention and W-to-apply are described in z_NETWORK.txt. Multicast group base is 239.255.0.0; the send socket and chk_send() are in network.c:165. A slave/master frequency-sharing protocol (SLAVE_MESSAGE, net_send_slaves_freq()) coordinates linked instances.
Related#
The tapped stages are the first FFT, noise blanker / TIMF2, second FFT and baseband of the main signal chain.